First, Make The Brownies:

1. Preheat Oven to 350 degrees F. Line an 8×8 metal baking pan with parchment paper & set aside.
2. Melt Chocolate & Better. In a small saucepan on the stove, melt butter and 2/3 cup chocolate chips over low heat until smooth and glossy. Set aside to cool slightly.
3. Whip Sugar & Eggs. In a large bowl, combine sugar, brown sugar, eggs, and egg yolk. Use a hand mixer to beat the eggs and sugar together 2-3 minutes, or until the mixture is lighter in color and the sugar is fully dissolved. (You shouldn’t feel any grit when you rub the mixture between your fingers.) Do NOT skip this step or your brownies won’t have a shiny, glossy top.
4. Add Chocolate Mixture. Slowly, mix the melted chocolate mixture into your whipped egg mixture, until smooth. Mix in the vanilla extract.
5. Add Dry Ingredients. Next, add gluten-free flour, cocoa powder, and salt. Gently stir until almost combined, with just a few dry spots left. Add the remaining 2/3 cup chocolate chips and gently fold in until well distributed.
6. Bake & Cool Brownies. Spread the brownie batter into the prepared pan and bake 25-30 minutes (mine are usually done at the 27-28 minute mark). Cool brownies COMPLETELY before frosting (otherwise the frosting will melt).

Next, Make The Frosting:

1. Start With The Butter. Add softened butter to the bowl of a stand mixer (or a large bowl). Using the paddle attachment (or a handheld electric mixer), beat the butter on medium-high speed for 2-3 minutes, or until very smooth and light in texture. It should look significantly lighter in color & have increased in volume.
2. Add Most Of Your Ingredients. Scrape down the sides of the bowl and add cocoa powder, powdered sugar, vanilla, salt, and 2 Tablespoons of heavy cream to the mixer.
3. Mix on LOW first. On LOW speed, pulse the mixer a few times so the powdered sugar and cocoa doesn’t poof out the sides of the bowl. (You can also add the dry ingredients in small amounts if you prefer to avoid messes)
4. Increase The Speed. Once it’s just mixed together, add 1/2-1 tablespoon more of cream, along with the corn syrup (if using). Increase the speed of the mixer to medium and mix for another 1-2 minutes, or until the buttercream is light and smooth. Ideally, you want it smooth and light without a lot of extra air bubbles.
5. Finish The Frosting. Use a flexible spatula to scrape down the sides of the bowl and press out any extra air bubbles, for best results At this point, the frosting should have a smooth, creamy texture and be easy to spread. It should hold its shape on a spatula if you rotate it around.
6. Spread with an offset spatula or pipe frosting with a piping bag and piping tip onto the cooled brownies, as desired.
7. Serve & Store. Enjoy right away or store leftover brownies in an airtight container in the fridge 3-4 days. Or, freeze brownies in a freezer bag up to 2 months.