Can I substitute unsweetened cocoa powder for unsweetened baker's chocolate?
Yes, that's a very standard substitution. For each square (ounce) of unsweetened chocolate in the recipe, use:


  3 tablespoons (20 grams) natural cocoa powder (not Dutch-processed) plus 1 tablespoon (14 grams) unsalted butter, vegetable oil or shortening


Joy of Baking

I'd add that Dutch processed cocoa (if that's what you happen to have) is fine too, just be aware of your acid balance. This is the opposite issue, but it's relevant here: Effect of undutched cocoa on baking

This too: What makes the difference between domed and flat cookies?

If your cocoa happens to be Dutch processed, ask another question about adjusting acid. Even though it usually doesn't matter, in that recipe, it just might.