Substitute Dutch processed cocoa

Use equal amount of dutch cocoa and add a little bit of baking powder. Since the dutch cocoa is neutral, you need a leavener like baking powder (if you are making any baked goods). Replacing the water with buttermilk will not do much unless there's a basic (as in alkali) counterpart, like baking soda. Sounds like you are making a cake, which means that you can add the same amount of dutch cocoa but you'll need baking powder which has an acid and base to create the leavening that you need for the cake to rise. Should be fine without altering the recipe too much. If you use buttermilk, you'll need to add some baking soda.

Look up a chocolate equivalent chart for how much cocoa, sugar and fat in the recipe can be replaced by bar chocolate (dark or semi-sweet).