Preheat The Oven & Prep The Pan. Start by preheating the oven to 325 degrees F. Line an 8x8" metal pan with parchment paper and set aside. (Use a METAL pan here, or the brownies won't bake the same)
Combine The Dry Ingredients. In a medium bowl, combine the gluten-free flour blend, cocoa powder, and salt. Whisk to combine.
Melt The Butter & Chocolate. In a medium saucepan, melt and 2/3 cup chocolate chips over low heat. Stir until the butter chocolate chips are melted and remove from heat. Let the mixture cool slightly while you mix up the eggs and sugar. (You can also melt the butter/chocolate in a microwave-safe bowl in 20-second increments, if preferred)
Whip Sugar, Eggs & Egg Yolk. In a large mixing bowl, whisk sugar, eggs and egg yolk 2-3 minutes or until lighter in color, about double in volume, and the sugar is mostly dissolved. (A hand mixer helps this job go faster!) Don't rush this step--it helps give your brownies the perfect fudgy, chewy texture & signature crackly tops you're looking for!
Mix Chocolate Mixture Into Egg Mixture. Slowly pour cooled chocolate mixture into the egg mixture, and mix in the vanilla extract.
Stir In The Dry Ingredients. Add the dry ingredients and stir until just a few small pockets of powder remain.
Fold In The Chocolate Chips. Add the second 2/3 cup chocolate chips, and fold in with a spatula until just evenly distributed and no more powdery spots remain. Transfer the brownie batter to your prepared pan and smooth the surface evenly.
Bake The Brownies. Pop the brownies in the oven and bake for 25-27 minutes or until the surface no longer jiggles in the center and the brownies look set around the edges. (Try not to over-bake, as this will dry the brownies out a bit.)
Cool Completely. Then, let the brownies cool completely. If they're even a little bit warm, it will melt the frosting. (Popping the pan in the fridge on a wire rack once it's cooled somewhat can help speed this step up)