This recipe was given to me in 1976 by a friend. I've changed it some to make it work for me. It has been so long I don't remember what changes I made but this is the way I make it now and have for 30 years. My kids love it. It makes a very impressive cake. It looks like it takes a lot of time but it doesn't really. By the time your layers are cool you can have everything else ready to go.
Mix cake ingredients and bake in 2 9" layer pans. 28 min. at 350* or till they spring back when touched.
Cool 10 min. in pans on wire rack then turn out to cool completely. When cool, slice each layer across center to make 4 layers.
While your cake is baking and cooling cook in a large boiler the filling ingredients except COCONUT over low heat. Double boiler works good unless you have heavy bottom pans. When your marshmellows melt add 1 pkg. of the coconut and about half of the second one. Just to thicken the filling. It needs to soak into the cake so it will be moist so don't make it too thick.
The filling will go between the layers and on top. Don't put it on the top layer till after you frost sides.
Frosting will go around the sides.
Melt butter with brown sugar and cocoa. Stir in 4 T milk and beat in powdered sugar with a hand mixer or hand blender. Use 2 T more milk if you need to make spreading consistency.
Frost around the sides only. If you can bring up a small lip at top edge it will keep the filling from running over the edge when you add it last.