2 cups all-purpose flour
2 cups white sugar
1 teaspoon baking soda
1/2 teaspoon salt
1/2 cup buttermilk (or sour cream)
2 eggs
1 cup butter
1 cup water
5 tablespoons unsweetened cocoa powder

Icing:

6 tablespoons milk
5 tablespoons unsweetened cocoa powder
1/2 cup butter
4 cups confectioner's sugar
1 teaspoon vanilla extract
1 cup chopped walnuts

Preheat oven to 350 degrees F (175 degrees C).

Grease and flour a 10x15 inch pan.

Mix the flour, sugar, baking soda and salt. Beat in the buttermilk (or sour cream) and eggs. Set aside. In a medium saucepan, melt the butter on low. When melted, add the water and 5 tablespoons cocoa. Bring to a boil then remove from heat. Allow to cool slightly, then stir cocoa mixture into the egg mixture, mixing until blended.

Pour batter into prepared pan. Bake at 350 degrees for 20 minutes, or until a toothpick inserted into the center comes out clean.

During baking, prepare the icing so it is ready to spread when the cake comes out of the oven:

Icing: In a large saucepan, combine the milk, 5 tablespoons cocoa and 1/2 cup butter. Bring to a boil, then remove from heat. Stir in the confectioners' sugar and vanilla, then fold in the nuts, mixing until blended. Work quickly to spread frosting over warm cake.