Brownies Preheat oven to 350°F. Line a 9×13 pan with parchment paper and coat with nonstick spray. You can bake these without parchment and coat the pan as well. Parchment paper makes it easier to lift the brownies from the pan to cut them. In a large microwave-safe bowl, combine the chopped chocolate and butter. Microwave on high heat in 30 second intervals, stirring after each until smooth. When the chocolate is melted, add in the sugar, eggs and salt and mix until smooth, using a wooden spoon or rubber spatula. Lastly, stir in the flour until combined
Pour the batter in the prepared pan and bake for 20-25 minutes until the center is just set and a toothpick inserted into it comes out with a few moist crumbs. Immediately sprinkle the marshmallows on top of the hot brownies and return to the oven for 2 minutes, allowing the marshmallows to puff slightly. Transfer the pan to a wire rack and prepare the frosting.
Frosting In a large saucepan, melt the butter, cocoa powder and milk together over medium heat until smooth. Remove from the heat and whisk in the powdered sugar until smooth and no lumps remain.
Pour the frosting evenly on top of the marshmallows, making sure all are coated. Allow the brownies to cool completely, 1-2 hours, before removing them from the pan and cutting them into bars.