Unsweetened chocolate is a mixture of cocoa solids and cocoa butter. Unsweetened cocoa powder starts out as unsweetened chocolate. Then the cocoa butter is forcibly removed, and the remaining solids are ground to a powder. So the difference between baking chocolate and cocoa is all that cocoa butter - generally as much as 55% of the baking chocolate.
When you substitute cocoa for unsweetened chocolate, you generally use 3 tablespoons of cocoa and 1 tablespoon of butter, oil, or shortening to replace 1 ounce of chocolate. Since you want to go in the opposite direction, you need to find a way to remove from your recipe the extra fat you are adding. For each ounce of unsweetened chocolate you use, omit 1 tablespoon of whatever fat is called for elsewhere in your recipe, if possible.
Depending on your recipe, you may also have to manage blending and mixing issues, as melted chocolate and cocoa do not necessarily have the same mixing properties at all ranges of temperature, but we're sure you can master that challenge.