Preheat & Prep. To start, preheat the oven to 350 degrees F. Line a METAL 9x5 loaf pan with parchment paper and set aside.
Melt Butter & Chocolate. In a microwave-safe bowl, combine butter and 1/3 cup chocolate chips. Melt on low heat in 15-20 second intervals, stirring in between each interval until the chocolate is melted, smooth, and glossy. Set aside to cool slightly.
Beat Egg, Sugar, and Vanilla. In a medium bowl, combine granulated sugar, egg, and vanilla. Whisk or beat with a hand mixer a FULL 2-3 minutes or until the sugar is almost completely dissolved. The mixture will get significantly lighter in color, and when you rub the mixture between your fingers you should barely feel any grains of sugar left. DO NOT rush this step, or your brownies will be gritty and won't have glossy tops.
Add Chocolate Mixture To Egg Mixture and gently stir to combine.
Add Dry Ingredients. In a small bowl, combine gluten-free flour, cocoa powder, and salt. Whisk to combine, then add to the wet ingredients and stir until just mixed. Add 1/3 cup chocolate chips and stir until evenly distributed.
Bake Brownies. Transfer the brownie batter to the prepared pan and smooth the surface. Bake at 350 degrees F for 15-18 minutes, or until the top looks shiny and set and the brownies don't jiggle when you gently wiggle the pan. (A toothpick inserted in the center should come out with just some moist crumbs, but not wet brownie batter)
Finish & Cool. Remove the brownies from the oven and sprinkle with flaky sea salt (if using). Let the brownies cool 5 or so minutes in the pan, then lift up with the parchment paper and transfer to a wire rack to cool completely before cutting. (This will help the brownies set up properly--you can speed this process up by chilling the brownies in the fridge)
Serve & Store. Cut brownies into 8 small squares, 6 medium rectangles, or 4 big rectangles. Store leftover brownies in an airtight container at room temperature 2-3 days, or freeze up to 2 months!