Add the butter to a small saucepan over medium heat. Cook, stirring or swirling occasionally, until the butter foams and the milk solids turn golden brown and smell nutty. Be careful not to let it burn. You should have roughly 10 ounces (280 grams) of browned butter.

Remove the butter from the heat and let it cool for two minutes.

Add the chopped dark chocolate and let it sit briefly to begin melting. Stir with a silicone spatula until completely melted and smooth.

Add the eggs, egg yolk, light brown sugar, granulated sugar, and vanilla to the bowl of a stand mixer fitted with the whisk attachment. Whip on high speed until the mixture is thick, pale, and increased in volume. This step helps create the brownies’ signature crackly crust.

While the chocolate mixture is still warm, gently fold it into the whipped egg mixture with a silicone spatula until just combined, scraping the sides and bottom of the bowl as needed.

Sift in the flour and cocoa powder. Gently fold until just incorporated, being careful not to overmix.

Preheat the oven to 350°F (180°C). Line a 9 x 9-inch (23 x 23-centimeter) baking pan with parchment paper.

Transfer the brownie batter to the prepared pan. Gently tap the pan against the counter to spread the batter evenly.

Sprinkle the top with flaky sea salt.

Bake for about 25 minutes, or until the edges are set and the center still has a slight wobble. For firmer brownies, bake for an additional five to ten minutes.

Transfer the pan to a wire rack and let the brownies cool in the pan for 30 minutes.

Using the parchment paper, carefully lift the brownies from the pan and transfer them to the wire rack to finish cooling.

Once cool to the touch, slice into eight portions. For clean cuts, dip a sharp knife into hot water, wipe it dry, and clean the blade between each cut.

Serve slightly warm for the best texture. Pair with good-quality vanilla ice cream and hot fudge sauce, if desired.