I found this recipe when I was making the Devils Food Cheesecake from Juniors that I found on the Food Network website. It is THE best chocolate icing I've ever tasted and it crusts really nicely. I used the Hershey's Special Dark cocoa powder.
6 cups sifted confectioners' sugar (1 1/2 pounds)
1/2 cup unsweetened cocoa powder
1/2 teaspoon salt
1 1/2 cups (3 sticks) unsalted butter, at room temperature
6 ounces bittersweet or semisweet chocolate, melted and cooled
2 tablespoons dark corn syrup
2 tablespoons pure vanilla extract
1/2 cup heavy or whipping cream
In a large bowl, sift the confectioners' sugar, cocoa, and salt together. In another large bowl, cream the butter with a mixer on high until light yellow and slightly thickened, about 3 minutes. With the mixer still running, beat in the chocolate, corn syrup, and vanilla. Reduce the mixer speed to low and beat in the sugar-cocoa mixture in two additions, beating well after each. Blend in the cream until the frosting is a spreading consistency, adding a little more cream if needed. Whip the frosting on high until light and creamy, about 2 minutes more.