You can substitute dutch process cocoa powder with regular cocoa powder in the cake, but with a slight change in leavening due to the acidity! Add the same amount of cocoa, but add 1/4 teaspoon of baking soda (for 3/4 tsp baking soda total). In the buttercream, you can substitute the cocoa type in the recipe with no changes, but the buttercream will be lighter in color.