Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan to prevent sticking.

In a large mixing bowl, combine the melted butter and sugar. Use an electric mixer to beat until the mixture is smooth and creamy.

Crack the eggs into the mixture one at a time, mixing well after each addition. Stir in the vanilla extract.

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

Gradually add the dry mixture to the wet ingredients, stirring until just combined. Be careful not to overmix!

Gently fold in the mini marshmallows and chocolate chips until evenly distributed throughout the batter.

Pour the batter into the prepared baking pan. Spread it evenly and bake for 30-35 minutes, or until a toothpick inserted in the center comes out with moist crumbs (not wet batter).

Once baked, allow the cake to cool in the pan for at least 10 minutes before slicing. Enjoy warm or at room temperature!