Preheat the oven to 350°F. In a large mixing bowl, cream together the melted butter and sugar until smooth. Beat in the eggs one at a time, followed by the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Grease and flour a 9-inch round cake pan. Pour the brownie batter into the prepared pan, spreading it evenly with a spatula.

Sprinkle the chocolate chips, mini marshmallows, and chopped walnuts evenly over the top of the batter, pressing them lightly into the surface.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking.

Allow the brownies to cool in the pan on a wire rack. Once cooled, cut into slices and serve. Pair with ice cream for an extra treat.

Store any leftover brownies in an airtight container at room temperature for up to 3 days. For longer storage, refrigerate or freeze in a sealed container.