You know, it's true what they say about baking. It truly is a precise science, and the final product is tremendously affected by the different chemicals you use. But when I first started baking, and had learned that there are multiple types of flours, I was skeptical still. I mean, flour is flour, right? But when you read into the scientific reasoning behind each ingredient, you start to see the bigger picture.
Just like cake flour isn't the same as bread flour, and baking powder isn't the same as baking soda, cocoa powder and baking cocoa are two entirely different things. Let's see how, and more importantly- can we swap one for the other? Spoiler- you can, but there's a trick to it.
While it smells like sweet chocolate, cocoa powder is bitter and acidic in taste. It is derived from fermented and roasted cocoa beans. These processes give the powder its tangy flavor, which is usually balanced out by using baking soda. The soda's alkalinity balances out the acidity of the cocoa powder, allowing the sweet flavors through. You will also need to use considerable amounts of sugar to balance out the bitterness.
Baking cocoa comes already treated in an alkaline solution to lower its acidity. While still bitter, it's not as bitter and tangy as raw cocoa powder, and it's more soluble in water, allowing it to melt into the batter efficiently. When using baking cocoa, you can use sugar more sparingly than you would when using cocoa powder.
All these traits are what make baking cocoa perfect for drinks and icing, unlike cocoa powder. Just as cocoa powder goes well with baking soda, baking cocoa goes well with baking powder.
Therefore, if you don't have baking cocoa on hand, you can use cocoa powder instead but follow this rule: for every 3 tbsp of cocoa powder, add 1/8 tsp of baking soda. This is to compensate for the acidity and ensure the rising of your baked goodies.
