Preheat oven to 325 degrees. Butter a 13-by-9-inch baking dish; line with parchment paper, allowing a 2-inch overhang. Butter lining, excluding overhang; set aside. Put butter, unsweetened chocolate, and 1 ounce semisweet chocolate into a heatproof bowl set over pan of simmering water; stir until butter and chocolates have melted. Remove from heat.
Whisk together flour and salt in a small bowl; set aside. Put sugar and espresso powder into the bowl of an electric mixer fitted with the paddle attachment; mix on low speed until combined. Raise speed to medium. Add chocolate mixture; mix until combined. Mix in eggs, one at a time, and vanilla. Add flour mixture; mix until just combined. Fold in remaining 6 ounces semisweet chocolate.
Pour batter into prepared dish; smooth top with an offset spatula. Bake until a cake tester inserted into center comes out with a few moist crumbs attached, 1 hour to 1 hour 10 minutes. Transfer dish to a wire rack; let cool 40 minutes. Lift out brownie by overhang. Return to rack; let cool completely. Cut into forty-five 1 1/2-inch squares.
Brownies may be refrigerated, wrapped in plastic, up to 1 week.
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