Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Mix until well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips, peanut butter chips, M&M's, and chopped peanuts if using.
Pour the brownie batter into the prepared baking pan, spreading it evenly. Sprinkle additional M&M's on top for decoration.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for at least 15 minutes before cutting into squares and serving.