Butter: I use salted butter in this recipe. If you use unsalted butter instead, increase the salt in the cake to 1/2 teaspoon and add a pinch or two of salt to the frosting.
Cocoa powder: I use regular unsweetened cocoa powder. I haven't tried Dutch-processed cocoa powder in this recipe.
Buttermilk: Regular or low-fat is ok. If you don't have buttermilk, you can make an easy substitute with this recipe.