I have a really good recipe for whipped vanilla frosting that I want to use, but I want to make a chocolate frosting. What kind of chocolate should I add to the frosting to make it a chocolate frosting? How much should I add? Should I reduce the amount of vanilla in the recipe or keep it the same?
Whatever your type frosting is, ie Italian meringue or confectioners' frosting, you can simply add enough (cooled) melted chocolate or cocoa powder (sifted to remove lumps) until it tastes good to you. Since neither cocoa powder nor melted chocolate will curdle the frosting it is simply a matter of taste. However, if you are going to refrigerate the frosting, remember that the chocolate will solidify and thus your frosting will become firmer.
For some rough guidelines I recommend 1/4 cup cocoa powder for every 2 cups of frosting and 3 ounces melted chocolate for every cup of frosting.
Use a good quality dark chocolate, with a high proportion of cocoa solids, for a strong chocolatey taste. Or, if you don't like dark chocolate, use a half and half mixture of milk and dark chocolate.
Real chocolate would just be diluted by the sugar/butter in your frosting recipe. You can use it (as much as you want), but I suspect you will be underwhelmed if you do. With the price of chocolate these days, you'll probably be disappointed with the outcome.
Cocoa powder is what you want. Add 1 tablespoon at a time until you reach the desired flavor profile.
If you want big chocolate flavor, consider a chocolate ganache (or other form of nearly-pure chocolate frosting.)
Marcel Desaulniers Death by Chocolate has plenty of recipes that won't let you down (but will cost a lot of money).