Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
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, 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 mixture, stirring until just combined.
Pour half of the brownie batter into the prepared baking dish and spread evenly.
In a small saucepan over low heat, melt the peanut butter until smooth.
Drizzle half of the melted peanut butter over the brownie layer, then sprinkle the mini marshmallows on top.
Pour the remaining brownie batter over the marshmallows and spread gently.
Drizzle the rest of the peanut butter on top and sprinkle with chocolate chips.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into 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.