What to do when a recipe calls for dutch process cocoa and you only have regular cocoa?

It's all about whether you should be using baking powder or baking soda. Cocoa is acidic, thus you don't need to use baking powder while using it, you can just use baking soda directly and they will react.

On the other hand, dutch-processed cocoa is almost ph-neutral you need to add some acidity for baking soda to react, which is provided through the tartaric acid in the baking powder; thus you should use baking powder instead of baking soda.