When/ how/ how much sour cream to cake recipe?

If you add sour cream when it isn't called for, you will probably ruin the cake and it will not it any more moist than liquid called for in the recipe. This sounds like a pretty moist cake recipe. Just start testing the cake about 15 minutes after you start baking the cake. Insert a tooth pick in the center of cake and when it comes out clean, the cake is done. 20 minutes is not that long to bake a cake.