CAKE: Preheat oven to 350 degrees. Grease and lightly flour a 10×15×1-inch jelly roll pan or baking sheet with deep sides. In a medium saucepan, bring butter, water and cocoa to a boil, stirring constantly. Remove from heat and set aside. In a large mixing bowl, combine flour, sugar, salt and baking soda. Add wet ingredients to dry ingredients. Using an electric mixer, beat to combine. Add in eggs, one at a time and sour cream, mix well. Pour into prepared pan and bake for 20-25 minutes.
ICING: Before cake is finished baking, in a small saucepan, bring butter, milk, and cocoa to a boil, stirring constantly. Remove from heat and add confectioners' sugar, vanilla and nuts, stir to combine. Frost cake immediately, don't wait for it to cool. Cool completely before serving.