Why didn't my milk curdle after adding vinegar?

Thanks! My milk was whole and not ultra. It did not seem different to me after a good 15 minutes. However, the cake was absolutely delicious!

I thought of something else. If your milk is ultra pasteurized, it may not thicken at all. The first time I made creme fraiche, I grabbed ultra pasteurized by mistake and it didn't work.

I let mine sit the full ten minutes and as I poured it, there were thicker areas. Sort of like blobs of soft yogurt. Did yours not do anything?

You usually need to heat the mixture for it to really curdle. For this recipe, you're only souring the milk which can thicken it slightly.