Let both the brownies and hot fudge cool for about an hour. Hot fudge brownie sundaes are wonderful with hot brownies, but if you take gluten-free brownies out of the pan too soon, they basically turn to mush. You need whole brownies for the sundae part to make sense.
Take the slightly warm brownie out of the pan and place it into a bowl. Cut it in half or even quarters so that the ice cream can sink down in between the pieces.
Add a scoop or two of vanilla ice cream on top of the brownie, and drizzle generously with hot fudge sauce. If you're feeling extra fancy, add some whipped cream, chopped nuts, and a maraschino cherry on top.