Step 1
Preheat oven to 350°. Grease a 13"-by-9" metal baking pan with cooking spray. Line pan with foil, leaving an overhang on 2 opposite sides. Grease foil with cooking spray.
Step 2
In a small pot over medium heat, melt butter. Remove from heat, add chocolate, and stir with a rubber spatula until melted and smooth. Stir in vanilla, espresso powder, and salt.
Step 3
In a large bowl, whisk granulated sugar and brown sugar. Whisk in chocolate mixture until smooth. Add eggs, one at a time, whisking to blend after each addition, then continue to whisk until batter is well combined, about 30 seconds more (this will help achieve a shiny, crackly top). Fold in flour and cocoa powder with a spatula just until combined.
Step 4
Pour batter into prepared pan. Bake brownie until puffed and set and a tester inserted into the center comes out clean, 22 to 28 minutes.
Step 5
Let cool completely in pan. Using foil overhang, remove brownie from pan and cut into squares with a sharp knife.
