Brownies with Marshmallows

I got this recipe from my mother-in-law.  It's a family favorite!  I just adapted it to make it gluten free.  I like the featherlight flour, but I'm sure any GF flour would work great.  My husband's coworkers are always nominating him to bring these brownies for work potlucks!  They're always a huge hit!

4 cubes butter
1/2 cup cocoa
3 1/2 cups Featherlight flour
3 1/2 cups sugar
2 teaspoons xanthan gum
1/4 teas salt
8 eggs
Mini marshmallows

Bring to a boil the butter and cocoa in a saucepan.  Take off the heat and let it sit for a few minutes.  Add GF flour, sugar, xanthan gum, salt and eggs.  Stir until smooth.  Bake in an 18 x 12 x 1 inch baking sheet at 375º for 20 minutes.  Cover the brownies with miniature marshmallows.  Put it back in the oven for a couple minutes to puff up the marshmallows.  Take out of the oven and let cool completely.  Top with chocolate frosting.  (The recipe below is the exact amount of frosting you'll need to cover the baking sheet.)

Chocolate Frosting
1/4 cup butter
3 cups powdered sugar
1/4 cup cocoa
milk

Beat butter, powdered sugar, and cocoa together, adding milk until it's the right consistency.