First, preheat your oven to 350 degrees Fahrenheit. Line an 8 inch square pan with parchment paper.

In a large bowl, combine the sugar, light brown sugar, melted butter, and cocoa powder. Stir vigorously for 2 minutes.

Then, add in the vanilla, eggs, and egg yolk. Stir to combine.

Add in the gluten free flour and tapioca flour. Stir until a slightly thick brownie batter forms.

Transfer the batter to the prepared pan. Smooth into an even layer.

Bake for 24 to 27 minutes, or until a toothpick inserted comes out with only a few moist crumbs.

Once the brownies have fully cooled, top with chocolate frosting and sprinkles. Refrigerate for 1 hour, or until the frosting has firmed up and set.

Finally, slice and serve!