Hersheys.com features a recipe for especially dark chocolate frosting and MarthaStewart.com has a recipe for dark chocolate frosting. The Hersheys.com recipe requires cocoa powder, while the MarthaStewart.com recipe requires both cocoa powder and semi-sweet chocolate.
To make Hersheys.com's especially dark chocolate frosting, melt 1 stick of butter or margarine in a saucepan and stir in 2/3 cup of Hershey's Special Dark Cocoa. Alternately add in 1/3 cup of milk and 3 cups of powdered sugar until frosting reaches a spreading consistency. Add a small amount of additional milk, if necessary. Stir in 1 teaspoon of vanilla extract. This recipe makes about two cups of frosting.
To make MarthaStewart.com's dark chocolate frosting, boil 1/2 cup plus one tablespoon of water and mix in 1/2 cup plus one tablespoon of unsweetened Dutch-process cocoa powder. Using an electric beater on medium speed, mix in 2 1/2 sticks of unsalted room temperature butter, 1/4 cup sifted confectioners' sugar and 1/4 teaspoon of salt until the mixture is pale and fluffy. Melt 1 1/2 pounds of high-quality semi-sweet chocolate in a saucepan or bowl in the microwave. Add the cooled melted semi-sweet chocolate to the cocoa mixture using an electric beater on low speed. Scrape mixture from the sides of bowl as needed. This recipe makes about five servings.