Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
Melt the butter in a saucepan over medium heat.
Remove from heat and stir in the sugar, mixing well.
Add the eggs one at a time, stirring vigorously after each addition.
Mix in the vanilla extract.
In a separate bowl, combine flour, cocoa powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet mixture.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 30 minutes or until a toothpick comes out clean.
Remove from the oven, top with mini marshmallows and chocolate chips, and let cool.