Preheat the oven to 350F/177C. Line a 8x8-inch baking pan with parchment paper. Grease the bottom and sides of the pan with nonstick cooking spray. Set aside.
In a medium bowl, whisk the gluten-free flour blend, cornstarch, baking soda, and salt together. Set aside.
In a separate large bowl, add the cocoa powder, hot melted butter, granulated sugar, and brown sugar together. Stir until the mixture forms a smooth paste. Let sit for 2-3 minutes to let the cocoa powder bloom.
Add the eggs and vanilla to the wet ingredients. Mix for an additional minute, until combined.
Add the dry ingredients to the wet ingredients. Mix until no more streaks of flour remain.
Fold in 3/4 cups (120g) of the chocolate chips, reserving the remaining 1/4 cup (40g) for later.
Spread the brownie batter into the prepared pan. Top with the remaining 1/4 cup of chocolate chips.
Bake at 350F for 22-25 minutes, rotating the pan halfway through. Bake until the edges of the brownies are set. A toothpick inserted into the center will come out mostly clean with some wet crumbs. Do not over-bake the brownies.
Remove from the oven and immediately sprinkle with fleur de sel (flaky sea salt).
Let the pan sit on a wire rack to cool. Use the parchment paper overhang to remove the brownies from the pan. Cut into squares.