To convert a cake recipe that uses bittersweet or semisweet chocolate to one using cocoa: (information taken from Rose Levy Beranbaum’s Cake Bible)
Substitute 1 tablespoon plus 1 3/4 teaspoons (9.5 grams) of cocoa, 1 tablespoon plus 1/2 teaspoon (14.5 grams) granulated white sugar, 1 1/2 teaspoons (7 grams) unsalted butter for every ounce (28 grams) of bittersweet or semisweet chocolate. Also, dissolve the cocoa in at least 1/4 cup (60 ml) hot liquid to bring out the cocoa’s full flavor.
To convert a cake recipes that uses unsweetened chocolate to one using cocoa: (information taken from Rose Levy Beranbaum’s Cake Bible)
Substitute 3 tablespoons (18 grams) cocoa plus 1 tablespoon (14 grams) unsalted butter for every 1 ounce (28 grams) of unsweetened chocolate. Dissolve the cocoa in at least 2 tablespoons of liquid in the recipe to bring out the cocoa’s full flavor.