Preheat the oven to 350°F. Grease a 9x13-inch baking pan or line it with parchment paper.
Melt the butter in a large mixing bowl, then stir in the granulated sugar until well combined.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
In another bowl, whisk together the flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour half of the brownie batter into the prepared baking pan, spreading it evenly.
Sprinkle the mini marshmallows over the batter, followed by the crispy rice cereal and M&M's.
Pour the remaining brownie batter on top, spreading it gently to cover the marshmallows and cereal.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before cutting into squares.