Texas Sheet Cake

Preheat oven to 350 degrees. Grease and lightly flour a jelly roll pan (12 x 17 – 1 inch deep)
In a medium saucepan over medium low heat melt 1 cup butter, add water and 4 tablespoons cocoa. Bring mixture to a low boil; whisking frequently. Remove from heat; cool for 20 minutes.
In a large bowl combine flour, sugar, baking soda and salt. In a separate small bowl whisk together the sour cream and eggs.
Pour the egg and sour cream mixture and the chocolate mixture into the flour mixture. Stir to combine mixing just until incorporated. Next pour into prepared pan and bake for 20-22 minutes or until toothpick inserted in center comes out clean.
In the last few minutes of the cake baking start the frosting. Combine milk, 1/2 cup butter and 4 tablespoons cocoa powder in a large saucepan over medium low heat. Bring to a low boil whisking constantly. Remove from heat. Working quickly stir in the powdered sugar and walnuts. Pour over the warm cake spreading with a spatula.