It's totally up to you - they will both work in this recipe! Dutch process cocoa powder (which you can reliably buy at Whole Foods) will yield a denser, darker loaf that won't rise quite as much but has a nice smooth and mellow chocolate flavor. Natural cocoa powder (aka Hershey's and most store brands) will result in a higher rise due to its natural acidity reacting with the baking soda. It will have sharper chocolate flavor, but a lighter color. The flavor will settle a bit as the loaf sits on the counter for a day. Both versions are delicious! Here's a photo showing the clear differences between the two - all other parts of the recipe were made with the same ingredients.