Makes about 1 1/2 dozen bars.
1 cup unsalted butter
14 ounces semisweet chocolate, coarsely chopped
4 ounces unsweetened chocolate, coarsely chopped
1/2 cup firmly packed dark brown sugar
1/2 cup firmly packed light brown sugar
2/3 cup granulated sugar
5 large eggs, lightly beaten
2 teaspoons pure vanilla extract
1 teaspoon milk, whole or 2 percent
1 1/2 cups unbleached, all-purpose flour
2 teaspoons instant espresso powder
3/4 teaspoon salt
1 3/4 cups coarsely chopped walnuts, toasted
1/3 cup heavy (whipping) cream
3 ounces bittersweet chocolate, finely chopped
2 ounces semisweet chocolate, finely chopped
Preheat oven to 350 degrees F. Line a 13 c 9-inch baking pan with aluminum foil, extending up and beyond the sides of the pan. Lightly (and gently so as not to tear it) grease foil.
For the brownie base: Combine the butter and chocolates in a large saucepan over low heat, stirring until mixture is melted and smooth. Whisk in sugars to blend.
Whisk in eggs, vanilla extract and milk, blending until smooth. Add flour, espresso powder, and salt just until mixture is combined. Stir in nuts. Spread into prepared pan.
Bake for 25 to 30 minutes or until a cake tester or wooden pick inserted in the center comes out with a few moist crumbs attached. Cool pan completely on a wire rack.
When cool, use foil ends to carefully lift brownies out of pan. Invert onto a clean cutting board, carefully remove foil, and then re-invert brownies. Return to a wire rack set atop a large piece of wax paper.
For the chocolate drizzle: Bring cream to a simmer in a medium saucepan. Remove pan from heat and add chocolates. Let chocolates melt in hot cream for 2 minutes without stirring. Whisk until smooth. Cool drizzle until thick but still soft and pourable. Pour over brownie base, allowing some to drip down and cover the sides. Spread drizzle with metal spatula, covering top and sides.
Let brownies firm up in the refrigerator. Cut into bars.