Prepare the Pan: Preheat the oven to 350°F and lightly coat an 8×8-inch baking dish with nonstick spray. Cut a piece of parchment paper so it covers the bottom of the dish and extends up and over the sides, creating a sling for easy brownie removal. Melt the Butter and Chocolate: Place the butter in a large saucepan and melt over medium heat. Add the 4 oz chopped chocolate and continue heating until melted.
Combine the Sugars: Turn off the burner and remove the saucepan from the heat. Whisk in the light brown sugar and granulated sugar until the mixture is smooth and creamy.
Add the Eggs: Whisk in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until evenly incorporated.
Mix the Dry Ingredients: Switch to a rubber spatula, then add the cocoa powder, flour, kosher salt, and espresso powder. Stir only until everything is just combined and no streaks of flour or cocoa powder remain.
Fold in the Chocolate: Gently stir the chopped dark chocolate into the brownie batter until distributed throughout.
Fill the Pan: Transfer the batter to the prepared baking dish, using the rubber spatula to scrape all of the mixture from the saucepan. Spread it into the pan as evenly as possible.
Bake the Brownies: Bake at 350°F for 35–45 minutes, or until the center is set and a toothpick inserted into the middle comes out with a few moist crumbs.
Cool Before Slicing: Place the baking dish on a wire rack and allow the brownies to cool before slicing.
Slice and Serve: Lift the cooled brownies from the pan using the parchment sling, then cut them into squares and serve.
