In a large bowl, whisk butter and cocoa powder together until cocoa is dissolved and mixture is thick. Stir in sugar until combined. Stir in eggs, mixing until combined. Add vanilla. Add flour, stirring just until no white streaks remain in batter. Fold in melted candy bars.
Pour batter in prepared pan and spread evenly. Bake for about 25 minutes, or until a toothpick inserted in the center comes out with fudgy crumbs, but not raw batter. Let cool slightly, about 20 minutes.
Spoon marshmallow creme/fluff over the top of the warm brownies and let it sit for a few minutes -- the warm brownies will soften the marshmallow, making it easier to spread. Spread marshmallow evenly over the brownies, then sprinkle the coconut over the surface, pressing gently with your hands.