Preheat your oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal.

In a large saucepan, melt the unsalted butter over medium heat. Once melted, remove it from the heat and stir in the granulated sugar until well combined.

Add the eggs, one at a time, mixing well after each addition. Then add the vanilla extract and stir until smooth.

In a separate bowl, combine the all-purpose flour, cocoa powder, baking powder, and salt. Gradually add the dry mixture to the melted butter and egg mixture, stirring until fully combined.

Gently fold in the mini marshmallows and chocolate chips until evenly distributed.

Pour the brownie batter into your prepared baking pan, spreading it evenly.

Bake in the preheated oven for 25-30 minutes, or until a toothpick comes out with moist crumbs.

Prepare the crunchy topping by mixing the crunchy rice cereal and an additional cup of mini marshmallows in a medium bowl.

When the brownies are done baking, remove them from the oven and sprinkle the topping mixture over the hot brownies, pressing down gently.

Return the pan to the oven and bake for an additional 5-10 minutes, until the marshmallows are puffy and golden.

Allow the brownies to cool completely in the pan, then lift them out using the parchment paper and cut into squares.

Serve on a decorative platter or individual dessert plates. Optional: dust with powdered sugar or drizzle with melted chocolate.