Preheat the oven to 350ºF (180ºC).

Choose an 8-inch (20cm) square pan or a 9 x 13-inch (22 x 33cm) pan. Line the pan with aluminum foil, leaving an overhang on two sides so you can lift the brownies out later. Grease the foil with the additional butter called for in the ingredient list or use nonstick spray.

Place the 1 cup (8 oz, 225 g) cubed unsalted butter in a small saucepan and melt over low heat. Remove from the heat and set aside to cool until tepid.

In a large bowl, sift together 1 cup (100 g) natural unsweetened cocoa powder, 1/4 cup (25 g) black (dark) cocoa, 1/2 cup (70 g) flour, 2 teaspoons instant espresso powder, and 3/4 teaspoon salt. Using a large bowl will help contain the cocoa dust.

In a separate small bowl, whisk together the 4 room-temperature eggs, 3/4 cup (150 g) granulated sugar, and 3/4 packed cup (170 g) dark brown sugar until the mixture is smooth and the sugars are mostly dissolved.

Add about half of the egg-and-sugar mixture to the sifted dry ingredients and stir until combined. Stir in the tepid melted butter. Then stir in the remaining egg mixture until the batter is smooth. If the batter isn’t fully smooth, give it a couple of vigorous stirs with a whisk, but avoid overmixing.

Scrape the batter into the prepared pan and smooth the top with a spatula. Sprinkle the surface liberally with flaky sea salt.

Bake in the preheated oven, starting to check at 20 minutes. Bake until the center is almost set but still slightly soft—about 25 minutes is common for either pan size. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter.

Remove the pan from the oven and let the brownies cool completely in the pan on a wire rack. Once cool, lift the brownies out using the foil overhang, transfer to a cutting board, and cut into pieces.

If desired, sprinkle a little more flaky sea salt over the brownies before serving.