A friend of mine asked me why most chocolate cupcakes recipes call for buttermilk but most vanilla call for regular milk. I have no idea. Does this make sense to anyone?
Generally if a recipe calls for baking soda, buttermilk is used as it activates the baking soda causing your cake to rise (baking soda alone has no leavening capabilities). Baking power contains baking soda & an acid and therefore doesn't need an acidic liquid to activate it. Simplified, recipes that call for baking soda call for some type of acid (buttermilk,sour cream, etc) and recipes that call for baking powder call for a neutral ingredient (milk, water, oil).
Not all chocolate cake recipes call for baking soda, as not all vanilla cakes call for baking powder. I've used recipes containing either/or, or both. HTH [/u]