In a heavy, large saucepan, melt the 1 cup butter and chocolate over low heat, stirring constantly. Remove from heat; cool.
Meanwhile, grease a 13x9x2-inch baking pan. Set aside. Stir sugar into cooled chocolate mixture in saucepan. Add the eggs, 1 at a time, beating with a wooden spoon after each addition just till combined. Stir in the 2 teaspoons vanilla.
In a small bowl, stir together the flour and baking soda. Add flour mixture to chocolate mixture; stir just till combined. Stir in coconut and nuts. Spread the batter in the prepared pan.
Bake in a 350 degree F oven for 30 to 35 minutes or till a toothpick inserted near the center comes out clean. Remove from oven. Place on a wire rack. Immediately spread the marshmallow creme evenly over the top. Cool completely in pan.
In a large mixing bowl, beat the 1/2 cup melted butter, cocoa powder, half-and-half, and the 2 teaspoons vanilla with an electric mixer on medium speed till smooth. Slowly beat in 4 cups of sifted powdered sugar. If necessary, beat in additional half-and-half till spreading consistency. Spread over cake. Cover; chill in the refrigerator up to 24 hours. Cut just before serving. Makes 15 servings.