Cocoa powder is the result of grinding cacao beans and then removing the fat, which is cocoa butter. If you're in the middle of making a recipe and have discovered you're all out, don't worry, you have some options.
Dutch-Process Cocoa vs. Natural Cocoa
There are two types of cocoa powder: Dutch-process cocoa and natural. Either can be labeled as "unsweetened cocoa." This just means that sugar hasn't been added to the final product, but it doesn't not tell you how to the cocoa was processed. (Cocoa powder in its natural state is not sweet.)
Natural cocoa tastes slightly more acidic, although you probably wouldn't think of it as acidic if you didn't try it side by side with Dutch-process cocoa. If your recipe does not specify which type of cocoa powder to use, you can use either.
Not sure which one you have? Natural cocoa tends to be lighter in color, almost a reddish brown, and also more intense in bitter chocolate flavor. When combined with the right amount of sugar, natural cocoa really hits that stimulating, deep chocolaty note.
Substituting Dutch-Process Cocoa Powder for Natural Cocoa Powder
If you have Dutch-process cocoa powder, you can add a little bit of acid to it (cream of tartar, white vinegar, lemon juice) to simulate natural cocoa powder. Replace the natural cocoa powder called for with an equal amount of Dutch-process cocoa. Then, bump up the acidity in the recipe by adding 1/8 teaspoon of cream of tartar, white vinegar, or lemon juice for every 3 tablespoons of cocoa powder used.
Substituting Unsweetened Baking Chocolate for Natural Cocoa Powder
Swap 1 ounce of unsweetened baking chocolate for 3 tablespoons of unsweetened cocoa powder. Then omit 1 tablespoon of butter, oil, or shortening from the recipe to account for the higher fat content in the baking chocolate. To make this substitution work, melt the unsweetened chocolate and add it to the recipe when you add the sugar and butter. This will ensure that it gets well integrated with the other ingredients.
Just know that your recipe probably won't taste quite as chocolaty as usual. Unsweetened cocoa has more cocoa solids, and that's where the flavor comes from. Your recipe will still taste good if you make it with unsweetened baking chocolate, but it won't have that off-the-charts chocolate flavor that the author of the recipe likely intended. And you may also notice other subtle differences in the texture of the finished product.
Can You Substitute Cocoa Mix for Cocoa Powder?
The short answer is yes, but it's definitely an extreme last resort. Cocoa mix contains only a small amount of cocoa powder, along with a lot of sugar, powdered milk, and who knows what else, so if you replace it measure for measure, you'll only be getting a small portion of the intended cocoa powder in the recipe, along with a bunch of extra ingredients. This will have all kinds of unintended negative effects on your recipe.