Bake chocolate cake mix in a 9x13 pan according to package directions.
Melt the marshmallows with milk, sugar and vanilla in a saucepan. Remove from heat and add coconut. Mix well and set aside.
Bring to boil 1 1/2 cups sugar, butter and 1/2 cup milk. Remove from heat and add 1 1/2 cups chocolate chips, stirring well until melted.
When cake is removed from oven, pour coconut mixture over cake, smoothing evenly, then pour chocolate mixture over this. Delicious!