Preheat oven to 350°F and lightly spray an 8x8” baking dish with nonstick spray. Trim a piece of parchment so it covers the bottom of the dish, but is long so it goes up and over the sides of the dish. This creates a “sling”, and will help you remove the brownies from the pan very easily.
In a large saucepan, add butter and melt over MED heat. Add 4 oz chopped chocolate and melt. Turn off the burner and remove pan. Whisk in brown sugar and granulated sugar, until smooth and creamy.
Add in eggs, one at a time, making sure to whisk well after each addition. Add vanilla and stir until combined.
At this point, you might want to switch to a rubber spatula. Add cocoa powder, flour, salt, and espresso powder, stirring until JUST combined and no flour or cocoa powder streaks remain. Gently stir in the chopped dark chocolate.
Pour batter into prepared baking pan, using the rubber spatula to scrape all the batter out of the saucepan. Bake for 35-45 minutes, or until center is set and a toothpick inserted in the center comes out with a few moist crumbs.
Let pan cool on wire rack before trying to slice. Then slice into squares and enjoy!