I usually opt for Dutch process cocoa powder (also called Dutched or Alkalized cocoa powder) in my cookie recipes. The process of alkalizing, which reduces the acidity, creates a darker colored cocoa and a deeper chocolate flavor. I have not tested this recipe with natural cocoa. I expect that it would produce a lighter, slightly cakier cookie due to the acidic reaction with the baking soda. For best results, stick with Dutch process cocoa powder for these cocoa cookies.