Baking is a science. Don't mess with it.

Greek yoghurt is much denser and dryer than regular yoghurt, perhaps you should've thinned it a bit more?

I generally sub plain yoghurt (not Greek style) without thinning it, but often add a couple/few of tablespoons more as necessary if batter is too thick. Water is also fine for thinning it, if desired, IMPE.

Greek yoghurt has more milk solids as well - hence the nearly double protein content, compared to regular.

Agreed about the yoghurt, but your substitutions don't sound so bad. Making up a new recipe takes a few tries to perfect, and that's what you were doing.

What temperature did you bake at? The number one culprit in dry cakes is baking too high. 325 degrees F is best with an untried batter.

Something I know from a lot of experience with improvisational cakes: if it looks like cake batter, and tastes like cake batter, and you didn't forget the baking powder, it's cake. You have cake.

So, to fix it, next time, make a simple syrup, maybe not flavored since you already had a lot of flavor in the cake, but one usually adds some fruit juice or liqueur. Use a small skewer to make holes throughout the cooled cake and brush the syrup on generously, a couple of times with at least an hour between.

I agree with @plllog, those substitutions are more or less OK. I regularly substitute Greek yoghurt instead of buttermilk. It works fine. But it does generally result in denser products because of all the extra protein. So, more leavening agent would be a good idea. You could also change the ratio of milk to yoghurt. Or maybe use a mixture of water and yoghurt instead.

You can also experiment with even more extreme substitutions. I have had great luck with using sourdough starter. I happened to have some excess one on hand. The tartness works really well as a substitute for buttermilk. And the starter is naturally light and fluffy when baked.

As far as moisture is concerned, yes, baking temperatures do play a big role. And so does fat and protein content. Try to match your original recipe.

In other words, substitutions are fine. But it might take one or two iterations to get things right. That's OK. Recipes often take trial and error to perfect. And a mango/banana cake sounds awesome.

I was just thinking that much of the Greek yoghurt I see on store shelves is fat free or low fat, which may also be a factor.

I always buy whole milk yoghurt and full fat milk.

Fat can inhibit gluten development and it can prevent starches from soaking up liquids. So, yes, getting the fat content right does make a big difference in the resulting product. Also, different types of fat have somewhat different outcomes. I frequently find that adding vegetable oil makes cake and breads more moist. So, that's something to experiment with.

And similarly, protein has a big influence on texture. It makes things less fluffy and more dense and maybe a little rubbery depending on the type of protein. And different types of proteins can be in all sorts of ingredients even if we don't normally think of them as protein sources.

Baking isn't really as impenetrable as people often make it out. I find, the only big difference to cooking is that with baking I can't really make adjustments by the time I know what the results are, whereas with cooking I can often make adjustments until the last minute. So, incremental iteration is a more labor intensive process. But it absolutely is possible. And with experience you'll get better at predicting which changes are going to have what effect ... nonetheless, I still get surprised every so often.

Carol, I actually double-checked to make sure the yogurt was full fat, also the milk. When I do desserts I think it's a little silly to try to cut the fat, after all, dessert is a seldom menu item around here and when we have it I want it to be just as sinful as possible! Again the flavor was excellent but I had no idea it was going to be dry. Also if I had thought to brush the layers with some rum it would have been nice but I was in a hurry to get it frosted and done since I still had to do dinner. The taste was so good I just might try this again and tweak it a little according to some of the suggestions offered here, I'll wait until I'm in a very energetic mood first though, it was quite a laborious bake. Well, that is, for me, not for a pro!

(Can you tell from my little comment that I'm intimidated by baking? ;-)

ᶜᵃⁿ ʸᵒᵘ ᵗᵉˡˡ ᶠʳᵒᵐ ᵐʸ ˡⁱᵗᵗˡᵉ ᶜᵒᵐᵐᵉⁿᵗ ᵗʰᵃᵗ ᴵ'ᵐ ⁱⁿᵗⁱᵐⁱᵈᵃᵗᵉᵈ ᵇʸ ᵇᵃᵏⁱⁿᵍ

If you want to write a "little comment", you have to write it small :-)

Seriously, though, I fully realize that many people consider baking to be intimidating. I think it has something to do with what you learned first. When I was young, my grandma first taught me how to bake, and only later did I learn how to cook. It changes perspective. I don't consider either activity intimidating.

I know that I occasionally make mistakes. And I know that trial'n'error sometimes works and sometimes fails spectacularly. But then, I am sufficiently confident in my cooking and baking skills overall, that I am not embarrassed to mess up every once in a while. And with that attitude, you'll discover that things are really not that bad.

Cooking has it's set of rules that most of us are probably familiar with. And baking does too. These rules just look different. For the types of dough or batter that I don't make frequently, I defer to recipes, but I might still make substitutions. But for things like bread dough or short bread, I have made them so many times, I would never bother looking at a recipe for more than just general inspiration.

It only takes three or four dozen attempts to bake a particular dough for you to have an excellent idea of how it has to feel and how it has to look when it is doing what it should do. And once you get to this point, you'll never forget. So, keep at it.

So true. And I was serious about the improvisational cakes. I've made scores. Once I only had one egg. It was denser than optimal, but it tasted good, and chalk it up to variety. I have a recipe (if you want a cake to come out exactly like it did previously, you do need to use a good recipe) for a perfect bundt cake. Perfect. But I rarely make it (not that I make many cakes nowadays because everybody is off sweets) because perfect becomes routine without variety. It's also a PITA, but that's kind of beside the point. If you have the basic ingredients in more or less the normal proportions, baking powder (unless it's an egg or yeast rising cake) and it looks and tastes like cake batter, it's cake.

Cookies are harder to mess with because they melt and spread, rather than simply rising. It's easier to stick to a basic recipe, like shortbread, sugar cookie or chocolate chip dough, and just change up the flavors and additions. Or read up on the principles, like a spoon of water is the difference between crisp and chewy, and butter will give you a melt in the mouth kind of crisp, whereas margarine will give you a tougher, chewier crumb (different melting temperatures and sizes of fat globules). The big rules for cookies (which great bakers may be able to mess with, but ordinary folks shouldn't): Always use cane sugar. If you use margarine, only use baking sticks with 100 Calories per TBSP. If you wait for the cookies to be toasty brown to remove them from the oven, they'll be overbaked by the time they're cool. Watch your moisture content for the desired outcome, especially if you have smaller egg yolks. The rest is whatever you feel like making...

M had the right of it, though. If you just do it and get to know the process, you'll get a feel for it and it won't be intimidating any more.

The real mistake is to stop trying.

I remember the 1962 Home Ec class that drilled it into us that baking was all about chemistry, I never got that lesson about cooking in general. To this day, I'll follow a cake recipe to a T, if I don't have an ingredient, I'll either shop for it or keeping looking for another recipe. Lucky for me, I've never been into bread and don't eat much of it, and I only do cakes or cookies on very rare occasions. Very few sweets at all in this house. So yes, baking is all mystery to me!

As a rule, I tend to agree with you sushipup, baking does seem to be precise chemistry and technique, more than other types of cooking. I like to bake but I am a stickler for have a T&T recipe and follow it to the letter. But once in awhile I go off the rails and try something not set in stone, mainly because I have ingredients that I don't want to waste and it usually doesn't work out very well. I love to make bread and cakes, not so much fancy pastries and baked desserts. I'm by no means an expert at it, but when I succeed I really like to eat it!

Yes, baking is chemistry, but once you have done a lot of it and know how different ingredients behave and which behave similarly, as plllog says, you can start playing with the recipes.