Step 1

Arrange a rack in the middle of the oven and heat the oven to 325ºF. Coat a 9x13-inch baking pan, preferably metal, with cooking spray. Line the bottom and two long sides of the pan with a sheet of parchment paper that extends a few inches over the sides to form a sling.

Step 2

Cut 2 sticks unsalted butter into 1/2-inch cubes. Coarsely chop 6 ounces bittersweet chocolate. Place both in a small saucepan and cook over low heat, stirring often, until melted. Turn off the heat. Add 1 1/2 cups granulated sugar and stir until the sugar is fully incorporated, about 15 seconds. Transfer to a large bowl. Let cool for 5 to 10 minutes.

Step 3

Add 3 large eggs and 1 large egg yolk one at a time, whisking well in between each addition. Continue whisking until the mixture is smooth and glossy. Add 2 teaspoons vanilla extract, 1 teaspoon instant espresso if using, and 1/2 teaspoon kosher salt, and whisk until combined.

Step 4

Sift 1/2 cup all-purpose flour and 1/4 cup cocoa powder into the bowl of batter. Gently fold with a flexible spatula until incorporated and no streaks of flour remain; do not overmix. Add 1 cup semi-sweet chocolate chips and fold until combined. Transfer to the baking pan and spread into an even layer.

Step 5

Bake until the center no longer jiggles, 25 to 30 minutes. Let cool completely in the pan. Grasping the excess parchment, lift the brownie slab out of the pan and place on a cutting board. Cut into 24 pieces.