Preheat oven to 350 degrees F (177 degrees C). Lightly butter a 4×6 or a 5×5-inch baking dish.
Stir together the melted butter and sugar in a medium-sized mixing bowl. Let completely cool.
Stir in the vanilla extract and egg.
In a separate small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Pour the dry ingredients into the wet ingredients and stir to combine.
Fold in half of the nuts the nuts.
Pour the brownie batter into the baking dish and top with the remaining chopped peanuts.Add more or less as you like.
Bake for 23 minutes. Remove from the oven. Sprinkle mini chocolate chips and mini marshmallow on top.
Return to the oven on a broiler setting. Broil for 3 minutes of until the marshmallow is lightly toasted. Do NOT leave un-attended when broiling as this could get burn easily.
Remove from the oven and let cool for 10 minutes. Slice and enjoy.