Preheat oven to 350°F according to cake mix box directions
Mix cake mix with 1 cup milk, 4 eggs, and 1 stick melted butter
Pour batter into a greased and floured 9" x 13" cake pan
Bake for 26-31 minutes until a toothpick inserted in the center comes out clean
Allow cake to cool completely
In a large saucepan over medium-low heat, combine ¾ cup milk, ¾ cup sugar, 24 marshmallows, and 14 oz coconut
Stir constantly until marshmallows melt and mixture is smooth
Pour marshmallow-coconut mixture over cooled cake and spread to cover evenly
Allow to cool for approximately 10 minutes
Place 2 cups chocolate chips and 1 stick butter in a microwave-safe bowl
Heat in 30-second increments, stirring after each, until melted (approximately 1-1.25 minutes total)
Spread melted chocolate mixture over the coconut-marshmallow layer
Allow to cool completely until chocolate sets up before serving
