SInce this is a bar cookie, you have much more leeway than if it had to do something other than lie in the pan. Made as written, it's going to be dense. There is no leavening.
The total sugar is 2 1/2 cups. Cutting it in half, to 1 1/4 cups, might be too much for taste; something other than the small amount of banana has to make the flour-and-oat mixture taste edible.
You could make it with 2 cups total sugar and I'm pretty sure it would be okay. Try 1 3/4. And I'd use more of the light brown. In fact, use the whole amount of light brown and then make up the rest with white. The brown sugar has moistness you'll need.

It has baking powder....that's leavening.
I would go whole hog and cut the sugar in half....a cookie with equal amounts of sugar and shortening/butter works pretty darn well.
With semi-sweet chocolate and 3 bananas it's going to be plenty sweet!
And it only has3 cups of oats and flour...I'd even go so far as to make it half oats and half flour....the reduced sugar will make it less liquid so it should work.
Linda C

A tablespoon of baking powder is a lot, isn't it? I think I'll combine your recommendations and cut the sugar in half but use all brown sugar and try half oats and half flour. How bad can it be? Closer to buttery granola and less like a cake.

Should I adjust the baking time then?

Thanks for your help. For some reason, this recipe kind of sang to me. Chocolate, oats and banana for breakfast. yum.

seagrass

Ah, sugar. It also affects moistness, texture, adds volume and contributes to browning. Conventional baking literature says you can cut by 1/4 without seriously compromising the final product.

However, I think you could cut it even more, for a couple of reasons. First, you are adding banana for moistness and sweetness, and the banana and the brown sugar are going to add the color so you won't need to worry as much about browning. The chocolate is also going to add sweetness. A tablespoon of baking powder is quite a lot, so it shouldn't be impossibly dense. If it is too dense and you want to try to "lighten it up" a bit , you could try whipping the egg whites and folding them in at the end.

I wouldn't adjust the baking time differently, I think you'll be OK, and if you don't like very sweet things, cutting the sugar in half is probably going to make it still plenty sweet.

I'd also probably take the "easy" way out and taste a bit of the batter to see if it was sweet enough at half the sugar. Yeah, yeah, I know, those raw eggs, and some people are uncomfortable with that, so it's your decision as to whether that's an option.

The recipe sounds really good, but quite sweet. I don't care for bananas, but I was thinking it would probably be good with applesauce too, or maybe pears.

Annie

A teaspoon of BP per cup of flour is pretty usual....
Bake it until it's done...:-)....browned...
If you have to break it up and call it cereal....so be it!

How did I miss the baking powder? Reading too fast, I guess.