Grease a 13x9x2-inch baking pan;* set aside. In saucepan, combine butter and water. Heat to boiling; stir to melt butter.
In large bowl, mix sugar, flour, cocoa, soda, vanilla, salt, and cinnamon. Add hot butter mixture; stir until smooth.
Add buttermilk and eggs, beating by hand until smooth.
Pour mixture into prepared baking pan. Bake in 350 degree F oven for 35 minutes or until a toothpick inserted in center comes out clean. Cool in pan on wire rack.
Spread Irresistible Frosting over cake and let stand at least 4 hours. Makes 12 to 16 servings.
You also can bake this chocolate cake in 2 greased and floured 9x1-1/2-inch round cake pans. Bake in a 350 degree F oven for 30 to 35 minutes. Cool for 10 minutes in pans on wire racks. Remove and cool. Frost cake as you like.
In saucepan, melt butter with marshmallows. Remove saucepan from heat. Add buttermilk or milk, cocoa powder, and vanilla. Stir in powdered sugar to make a thin frosting. Stir in chopped nuts. Pour over cake, spreading the frosting evenly.