Preheat your oven to 325°F. Line an 8-inch square light-colored metal pan with parchment or nonstick foil. Set aside.
In a small saucepan, melt the butter and sugar over medium heat, stirring occasionally, until the mixture begins to bubble. Remove from the heat and let cool slightly.
In a large bowl, whisk the eggs and vanilla until well combined. Slowly drizzle in the warm butter-sugar mixture, whisking constantly.
Whisk in the cocoa powder until smooth.
Switch to a spatula and stir in the flour blend, xanthan gum (if using), cornstarch, baking soda, and salt.
Fold in about 4 oz of the chocolate chips. The batter will be thick and sticky.
Spread the batter evenly in the prepared pan and smooth the top. Sprinkle the remaining chocolate chips over the top.
Bake for about 30 minutes, or until the center no longer looks wet.
Cool the brownies in the pan at room temperature. For clean slices, chill the pan in the refrigerator for at least 1 hour.
Lift out the brownies, slice into 9 squares, and serve—or wrap and freeze for later.