For the Cake: Preheat oven to 350 degees. Grease and flour three 9" cake pans. (Or see recipe #452719.).
Cream butter and sugar in large mixing bowl until very light and fluffy (this could take 5-7 minutes). Add the eggs one at a time, making sure to beat well after each addition and scraping sides of bowl. Blend in vanilla and chocolate.
Sift together the flour, baking powder, baking soda and salt. Add 1/4 of dry ingredients to creamed mixture then approximately 1/4 of buttermilk alternating remaining ingredients just until mixed. Pour batter into prepared cake pans; batter will be thick.
Bake in preheated oven for about 25-30 minutes until pick in center comes otu clean. Cool. (Mine took 27 minutes.).
For the filling: In a saucepan cook milk, sugar and marshmallows until melted. Add coconut; spread between layers.
For the Chocolate Topping: Melt together the butter, chocolate and marshmallows. Beat in confectioners' sugar and vanilla. Add enough milk to spread. Spread over cake.