To tell you the truth I don't think that you even need to thin sour cream to sub for creme fraiche, although that is fine. I make creme fraiche lots for various recipes and in my view it is it's tarter taste that sets it apart from sour cream more so than the consistency. I think in most recipes you can sub sour cream for creme fraiche with no ill effects at all to the recipe. Mind you creme fraiche does have a certain "je nais c'est quoi" and is well worth the effort.

I agree with cloudy christine - you lacked live bacteria (aka cultures) for fermentation found in buttermilk. You can use reconstituted powdered buttermilk for this purpose if you don't want to purchase liquid buttermilk. Plain yogurt (with live cultures) would also work as a source for live bacteria necessary. If you try again, you might want to use your oven with the light on for a more constant temperature, and the temperature needs to be around 80-degrees F for this type of fermentation. Be sure to give the oven about 30-minutes to warm-up and take the temperature at several spots in the oven. My "best" temperature is usually as far from the light as possible. -Grainlady

Here's the recipe as I copied and pasted years ago: This is from the March 2004 issue of Food and Wine magazine. I've made it for guests on several occasions and it has always been well received. It's quick, simple, and very tasty. Can be made ahead, too: Frozen Lemon Cream Sandwiches one 7-oz container chilled creme fraiche 1/4 cup lemon curd finely grated zest of one lemon 12 crisp butter waffle cookies 1/4 cup chopped unsalted pistachios Line a small baking sheet with wax paper. In a chilled bowl, beat the creme fraiche with the lemon curd and zest until firm peaks form. Arrange half of the cookies on the baking sheet and spoon the lemon cream onto the centers, letting it ooze gently to the edges. Top w/the remaining cookies, pressing down very gently. Transfer the baking sheet to the freezer and freeze the sandwiches until firm, at least 4 hrs. Spread the nuts on a plate and roll the edges of the sandwiches in them. 6 servings NOTES: Jules Destrooper Crisp Butter Wafers (from Belgium) are available from specialty shops in the US. They are, as their name implies, wonderfully crisp and buttery ... accept no substitute!

You can't use sour cream to make creme fraiche in half and half...in fact you need to add 1/2 c sour cream to whole cream and that will give you creme fraiche. The problem with your formula is you can't start with 1/2 and 1/2 ....I am so sorry. As to the buttermilk. You only need a small amount so you can just get a container that is a pint if your store carries it. Also yes it does freeze but it separates when it is defrosted so you have to just cook with it ...no problem though.Also it keeps for months in the fridge...you can ignore the "use by" " sell by" date...it lasts and lasts. You can just freeze the 1/2 and 1/2 also. I do it all the time. It too will separate but that is OK if you are going to cook with it. Or use it to make homemade yogurt...will be very very rich and yum...or make ice cream or rice pudding...lots of things...make a tres leches cake !! I am SO sorry you are not feeling well. I hope you are better tomorrow. c