I tried to use fromscratchSF's (beyond buttercream now) banana version of this recipe, substituting the roasted bananas for the sour cream and increasing to 2 tspns each baking soda and baking powder. I was attempting cupcakes. They puffed up really high at first, then fell and turned dark brown on the outside all around the cake. The inside was delicous and fluffy but they were not asthetically pleasing at all. Any suggestions on how to get the cake to bake up light on the outside and not fall?