Here;s two simple and easy recipes we've used for a long time:
In a saucepan, combine 1 cup sugar, 1/2 cup cocoa powder, milk, butter and 1 teaspoon vanilla extract. Bring to a boil, and cook for 1 minute. Remove from heat. Using an electric mixer, beat icing for 3 minutes, or until it thickens to spreading consistency. Pour over completely cooled cake.
OR:
Devil�s Fudge Icing
1 1/2 cups unsweetened cocoa powder
1/2 cup (1 stick) unsalted butter, at room temperature
1/2cup light corn syrup
1 teaspoon vanilla extract
1/2 teaspoon salt
1/2 cup hot water
2 1/4 cups confectioners� sugar, sifted
In a stand mixer fitted with a paddle attachment, cream together the cocoa powder and butter on low speed until blended, scraping down the bowl with a rubber spatula as needed, until very smooth, about 2 minutes. Add the corn syrup, vanilla extract, and salt and continue to mix until evenly blended, 1-2 minutes. Add the hot water and mix on low speed until evenly blended.
Add the confectioners� sugar, mix on low speed until blended, then increase the speed to medium and whip until the icing is very smooth and light, 2-3 minutes. Use immediately or transfer to a storage container, cover tightly, and refrigerate for up to 1 week