Here's What You Can Substitute For Cocoa Powder
When you're out of cocoa powder, but you really want to make a chocolate cake, there are many options available. They do, for the most part, require other types of chocolate, as cocoa powder isn't as versatile in regard to substitutes as other ingredients.
The best and easiest thing to use in place of cocoa powder is unsweetened chocolate. For every 3 tablespoons of cocoa powder required, use 1 ounce of unsweetened chocolate. The chocolate should be melted before being added to the recipe. Additionally, because the chocolate contains more fat than cocoa powder, reduce the fat in your recipe, such as shortening or butter, by 1 tablespoon (via HuffPost).
Another option is Dutch-process cocoa. However, you'll need to boost the acidity in your recipe. To do so, add 1/8 of a teaspoon of cream of tartar, lemon juice, or white vinegar (via The Spruce Eats).
Additionally, unsweetened or semi-sweetened chocolate chips are an option. They are likely the kind you buy to bake chocolate chip cookies.
Other cocoa powder substitutions
Just as with unsweetened chocolate, the chocolate chips will have to be melted before being added to the recipe. Also, the semi-sweet chips do contain added sugar. Compensate for this by reducing the amount of granulated sugar or other sweeteners you add. It is also a good idea to slightly reduce the butter or shortening to compensate for any cocoa butter in the chocolate (via Spiceography).
Carob is the best non-cocoa substitute. The flavor isn't quite the same, but it's close enough that it works as an alternative, and it does have a similar appearance to chocolate. Use carob powder measure for measure to the cocoa powder.
Cocoa mix, which seems like it would be a good substitute, should only be used as a last resort. Cocoa mix only has a small amount of cocoa powder in it, so measure for measure, you will be adding less cocoa powder to your recipe, as well as the addition of whatever else may be in the cocoa mix, like a ton of sugar, and it can be hard to figure out exactly how much to add.