Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking pan.
In a large bowl, stir together the melted butter and sugar. Beat in the eggs one at a time. Stir in the vanilla extract and salt.
In a separate bowl, combine the cocoa powder, flour, baking powder and baking soda. Gradually stir the dry ingredients into the wet mixture until all ingredients are combined.
Stir in 1 cup of the chocolate chips. Spread the batter into the prepared pan.
Sprinkle the marshmallows and remaining 1/2 cup of chocolate chips over the top. Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
Allow to cool before cutting into bars.
Interesting Facts
Marshmallows have been around since the ancient Egyptians!
Brownies are believed to have originated in the United States in the late 1800s.