In a small bowl, pour water over chocolate; set aside.
In a large bowl, cream butter and sugar until light and fluffy. Add egg yolks, one at a time, beating well after each. Add cooled chocolate and vanilla. Combine the flour, baking soda and salt; add to creamed mixture alternately with the buttermilk.
In another bowl, beat egg whites until soft peaks form. Fold into batter. Grease and flour one 8-in. round baking pan and one 13-in. x 9-in. baking pan. If desired, line bottoms with waxed paper; grease and flour paper.
Pour 2-1/2 cups batter into round baking pan. Pour the remaining batter into the rectangular pan. Bake at 350° for 25-30 minutes (round) and 35-40 minutes (rectangular) or until a toothpick comes out clean. Cool in pans for 10 minutes before removing to wire racks. Remove waxed paper and cool completely.
For frosting, combine sugar and cornstarch in a saucepan. Stir in milk and egg yolks until smooth. Add butter; bring to a boil over medium heat until thickened and bubbly, stirring constantly. Boil for 2 minutes. Remove from the heat; stir in vanilla and coconut.
Cool completely; set aside 3/4 cup. Tint remaining frosting green. Place rectangular cake on serving platter; frost top and sides with green frosting. Place 1/2 cup vanilla frosting in a decorator tube; pipe yard lines across cake.
For football, slice a 2-in.-wide strip from the center of round cake (save for another use). Spread reserved coconut frosting between layers of cake, forming the football. Place cut side down on a flat surface.
Combine remaining vanilla frosting with cocoa; frost top and sides of football. Use decorator tube to pipe laces on football. With a large spatula, carefully lift football and place on the green cake. Use three pipe cleaners at each end of cake to form goalposts. Yield: 20-24 servings.