Preheat your oven to 350°F.

Line a 15x10x1-inch baking sheet with foil, then spray the foil with non-stick cooking spray.

In a mixing bowl, cream together the softened butter and granulated sugar until just combined.

Add the eggs one at a time, mixing after each addition. Stir in the vanilla extract until incorporated.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until well combined. Spread the batter evenly in the prepared pan and bake for 15-18 minutes, or until the edges are set.

Once the cake is out of the oven, immediately spread the marshmallow fluff over the warm surface. The heat will make it easier to spread evenly. Let the layer cool for about 30 minutes.

To prepare the topping, combine the chocolate chips, miniature marshmallows, and butter in a saucepan. Melt the mixture over medium heat, stirring until smooth.

Once melted, mix in the rice crispy cereal until evenly coated.

Spoon the topping over the marshmallow layer, spreading it quickly and evenly before it begins to set.

Chill the pan in the refrigerator until the topping is firm.

Once set, slice into bars and serve. Store the bars in an airtight container at room temperature.