• Preheat the oven to 350°F. Line an 8x8-inch square metal baking pan with parchment paper leaving a 2-inch overhang at all sides.
• Place eggs, brown sugar, vanilla and salt in a large bowl (or the bowl of a stand mixer) and mix for 30 seconds on medium speed using an electric handheld mixer. Gradually stream in granulated sugar and continue to mix on medium speed for 3 minutes until creamy and pale yellow. Do not mix on high speed because the goal is to dissolve the sugar and build some structure, but we do not want to introduce large air bubbles. Mix in espresso powder if you're using it and set aside while you prepare the cocoa mixture.
• Place remaining 5 tablespoon (70g) of butter, cocoa powder and powdered sugar in a medium heatproof bowl and set aside.
• Brown some of the butter. Place remaining ½ cup (113g) of the butter in a saucepan and follow the instructions in this post here to learn how to brown butter. It’s easy! Melt the butter gently over medium heat in a saucepan and and continue to cook until a dense foam forms at the surface and the milk solids appear a golden brown specks.
• Pour hot brown butter into the bowl over the butter, powdered sugar and cocoa and whisk until the mixture is smooth. The heat will melt the rest of the butter and also melt some powdered sugar to encourage the formation of a shiny top on the baked brownies. If it is really cold in your house and the mixture isn't smooth, place the bowl in the microwave for 20-30 seconds until completely melted and glossy.
• Add the still-warm cocoa mixture to the whipped egg mixture and mix it in using a spatula until smooth and glossy. The batter should tighten up and that’s how you know the fat is emulsified. Sift the flour over the batter and fold it in gently until evenly combined. Stop mixing as soon as you don't see any dry pockets of flour and be careful to not over-mix. It should be smooth and glossy, like thick hot fudge.
• Pour the batter into your prepared pan and use a small offset spatula to smooth out the surface. Tap the pan on the counter to bring any large air bubbles to the surface, then run the spatula over the top to pop them. Bake for 25-28 minutes* or until a toothpick inserted in the center comes out with moist sticky crumbs attached. Make sure you do not over-bake these or they will be dry. Transfer pan to a wire rack and let brownies cool completely in the pan. Refrigerate for at least 1 hour before lifting out and cutting into squares.

NOTE: If your oven runs hot, try baking at 325°F for 30-35 minutes which will help prevent the edges from over-cooking before the center is set.