Preheat the oven to (350°F)= (176°C) degrees. Grease the 1/4 sheet cookie sheet. Combine the flour and sugar in a bowl. Bring the butter, cocoa, and water to a boil. Turn the stove off. Add the buttermilk, soda, egg, and vanilla extract to the pan with the boiling mixture. Use a hand mixer or electric mixer to combine the dry ingredients flour and sugar in the pan, blend until smooth. Pour into a greased pan. Baking time: 1/4 sheet jelly roll/cookie sheet, 20-25 minutes (when a toothpick comes out clean)
Grab a medium saucepan and melt the butter, add the cocoa and the milk. Bring to a boil while you stir or whisk constantly. Add the powdered sugar, vanilla extract, and nuts. I make this frosting while the cake is baking and then spread it on the semi-cooled cake.