Preheat your oven to 350°F (175°C) to ensure even baking.
Grease your 9x13-inch baking pan with butter or line with parchment paper.
Cream together the granulated sugar and softened butter until light and fluffy, about 3-5 minutes.
Beat in the eggs one at a time, then add the vanilla extract and mix until combined.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt. Gradually add this to the wet ingredients, stirring until just combined.
Gently fold in the mini marshmallows until evenly distributed.
Spread the batter evenly in the prepared baking pan.
Bake in the preheated oven for 25-30 minutes, or until a toothpick comes out clean.
Melt the chocolate chips and 3 tablespoons of butter, then mix in the peanut butter until smooth.
Once the bars are baked, pour the chocolate peanut butter mixture over the top and spread evenly.
Sprinkle the Rice Krispies over the chocolate layer, pressing down slightly.
Allow the bars to cool in the pan for at least an hour before slicing into squares.
