Step 1: Prep Your Pan. Preheat your oven to 325°F. Line an 8-inch square baking pan with parchment paper or nonstick aluminum foil. If using regular foil, spray lightly with nonstick cooking spray. Set the pan aside.
Step 2: Melt the Butter and Sugar. Combine the butter and sugar in a small saucepan over medium heat. Whisk occasionally until fully melted and just starting to simmer. Remove from heat and let it cool until it's no longer hot to the touch. (To speed this up, set the bottom of the pan in a bowl of cool water - just don't let the butter solidify.)
Step 3: Mix the Wet Ingredients. In a large bowl, whisk together the eggs and vanilla until well combined. Slowly pour in the cooled butter-sugar mixture while whisking constantly. Add the cocoa powder and whisk until fully absorbed. Use a handheld mixer to beat everything together until very smooth and glossy.
Step 4: Add the Dry Ingredients and Chocolate Chips. Add the gluten-free flour, xanthan gum, cornstarch, baking soda, and salt. Stir with a silicone spatula until just combined - don't overmix. Fold in about 4 oz of the chocolate chips (reserving the rest for the top) until evenly distributed.
Step 5: Fill the Pan and Bake. Pour the batter into your prepared pan. Use a lightly moistened spatula to spread it into an even layer. Scatter the remaining chocolate chips over the top and give the pan a firm bang on the counter to help them stick. Bake in the center of the oven for about 30 minutes, until the edges are set and the center no longer looks wet. Note: the toothpick test isn't reliable here - trust your eyes.
Step 6: Cool, Slice, and Serve. Let the brownies cool in the pan until the pan is no longer hot. For the cleanest slices, transfer the pan to the refrigerator and chill for about 1 hour. Remove from the pan, peel away the parchment, and cut into 9 squares. Serve immediately or wrap individually and freeze for later.
