Preheat oven to 350 degrees. Lightly spray a 9 x 13" pan with non-stick baking spray. Set prepared pan aside.

In a bowl, combine the flour, sugar and salt. Whisk together to combine. Set aside.

To a mixing bowl add the melted butter and cocoa powder. Using a hand or stand mixer, mix the butter and cocoa until completely combined and smooth. Add in the eggs and vanilla and mix well.

Gradually add the dry ingredients to the wet ingredients a few spoonfuls at a time. Stop the mixer occasionally to scrape down the sides of the bowl. Once all ingredients are combined, the batter will be very thick.

Spread the batter evenly into the prepared pan. Bake at 350 degrees for 25-28 minutes or until a toothpick inserted to the center of the brownies comes out clean and crumb-free. Remove from oven and let brownies cool completely to room temperature before frosting.

For the frosting: in a sauce pan, add the sugar, marshmallow, chocolate chips, milk and butter. Heat over medium-low heat, stirring frequently. Once all the ingredients have melted together, remove from heat. Let the frosting cool for 3-5 minutes before pouring over the brownies. Let the frosting set up and cool completely, for at least an hour. Once frosting is set, cut brownies into 20 squares.