Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper. Be sure to leave about an inch of parchment paper above the edges that you can use to lift the brownies out of the pan before slicing. Lightly spray the parchment paper with nonstick cooking spray.

Add the flour, unsweetened cocoa powder, and salt to a medium size mixing bowl. Whisk to combine.

Add the melted butter, granulated sugar, and vanilla extract to a large mixing bowl. Whisk to combine.

Whisk in the beaten eggs.

Stir in the flour mixture. Stir just until incorporated. (Be sure not to overmix)

Evenly spread the brownie batter in the prepared baking dish. Bake for 25-28 minutes, or until a toothpick inserted comes away somewhat clean with just a few crumbs.

Remove the brownies from the oven. Evenly sprinkle the miniature marshmallows over the top of the brownies.

Add the melted butter, unsweetened cocoa powder, half and half, and vanilla to a medium-sized mixing bowl. Whisk until smooth.

Add the sifted powdered sugar 1 cup at a time, whisking well after each addition until smooth. Evenly spread the frosting over the marshmallow layer. Cover and chill in the refrigerator for 2 hours.

Just before serving, lift the brownies out of the pan using the parchment paper. Slice the brownies 3 slices x 5 slices = 15 slices.