Over medium heat, melt butter in a large skillet or ovenproof frying pan (approximately 22cm base and 28cm top). Stand for 10 minutes to cool slightly.
Preheat oven to 180 degrees C or 160 degrees C fan forced.
Add sugars to skillet and whisk until combined. Add egg and whisk to combine.
Sift flour and bi carb soda over pan. Stir until combined.
Add desiccated coconut and peanuts. Stir until combined. Press down with spatula to level dough.
Bake for 17 minutes, or until skewer inserted in centre comes out clean.
Take cookie out of the oven and top with marshmallows. Sprinkle with coconut.
Place under hot grill for 1 to 1 1/2 minutes, or until marshmallows are golden.
Drizzle melted chocolate over top. Best served warm. You can cut into individual wedges or give each person a spoon to eat straight from the pan.