"Can unsweetened cocoa powder be used instead of unsweetened or semisweet chocolate squares or chips? If so, how do you determine measurements?"
To replace one ounce of unsweetened chocolate, use 3 tablespoons cocoa powder plus 1 tablespoon butter, shortening or oil. Dissolve the cocoa in liquid that is already used in the recipe.
To replace one ounce of semisweet chocolate, use 3 tablespoons of cocoa plus only 1 1/2 teaspoons of butter, shortening or oil and 1 to 1 1/2 tablespoons of sugar.