So if I need 2/3 of a cup of cocoa powder and I don't have it how much semi sweet chocolate squares do I need in place of the cocoa
One ounce of semi-sweet chocolate equals 3 tablespoons cocoa powder + 1 tablespoon butter or oil + 1 tablespoon of sugar (from The Cook's Thesaurus)
2/3 cup = 10 2/3 tablespoons
If your recipe calls for 2/3 cup cocoa powder and you want to substitute semi-sweet chocolate, you will need to reduce the butter or oil in your recipe by 3.5 tablespoons and the sugar by 3.5 tablespoons (10 2/3 divided by 3 equals 3.5)
You will also likely incorporate semi-sweet chocolate differently than you would cocoa powder, depending on what it is you are making. For example, for brownies, instead of mixing the cocoa powder directly in as in Alton Brown's recipe for Cocoa Brownies, you may need to melt the chocolate in a double-boiler, let it cool, then proceed with instructions such as those for Ghirardelli's Classic Chocolate Brownies that calls for semi-sweet chocolate.