Mix flour, sugar and soda in bowl. Bring butter, water and cocoa to a boil in saucepan. Pour over dry ingredients. Mix thoroughly. Add buttermilk, eggs, and vanilla. Beat well. Pour into a greased 9x13 pan and bake at 350 degrees for 30 minutes or until it tests done. To make frosting: heat butter, cocoa, and buttermilk to boiling. Remove from heat and mix in vanilla, sugar and nuts. Note: A delicious, moist, chocolate cake. The frosting will be runny but it hardens as it cools. Cake freezes well. Good with peppermint ice cream. It is also good with a 7-Minute frosting.