Put the cocoa in the hot water and set aside until last.
Mix the first six ingredients together.
Add the buttermilk mixture to the first six ingredients.
Add the cocoa and hot water mixture.
Bake in a greased and floured 9x13 pan for 30 to 35 minutes at 350 degrees. Cool in pan.
Mix the margarine, light brown sugar, and canned milk in a saucepan. Bring to a boil. Remove from heat and beat until creamy. (I poured the hot frosting into a cool bowl and placed it into a larger bowl containing cool water and ice to cool the frosting down faster.) Frost cake.