Beat the egg and 3 tablespoons of margarine, then add the cake mix and milk and beat for about 2 minutes or according to the package instructions, then place in a greased pan and let it rest while you make the filling.
In a pan put 1 tablespoon of margarine condensed milk and cream and the grated coconut and mix on low heat until it thickens and unglues from the bottom.
Then put it on top of the cake, and put it to bake at 180 degrees for 40 to 50 minutes (depends on your oven), wait for the toothpick to know if it's ready