1 cup unsalted butter, cut into tablespoons
1 cup packed dark brown sugar
2/3 cup granulated sugar
3 cups semisweet chocolate chips
2 ounces unsweetened chocolate, coarsely chopped
5 large eggs (at room temperature)
1 tablespoon vanilla extract
1 1/2 cups all-purpose flour
2 teaspoons instant espresso powder
1/4 teaspoon salt
1 cup walnut halves

Position a rack in the center of the oven and preheat to 350 degrees F. Line a 13 x 9-inch pan with aluminum foil so that the foil extends 2 inches beyond the two long sides of the pan. Butter the bottom and sides of the foil-lined pan.

In a medium saucepan, combine butter and sugars. Cook over medium heat, stirring constantly with a wooden spoon until the butter is melted and the mixture is very hot. Place the chocolate chips and unsweetened chocolate in a food processor fitted with the metal chopping blade. Process the chocolates for 15 to 20 seconds or until finely chopped. With the motor running, pour the hot butter mixture through the feed tube and process for 15 to 20 seconds or until the chocolate is completely melted, scraping down the work bowl as necessary. Add the eggs and vanilla extract and process for 10 to 15 seconds, or until combined. Add the flour, espresso powder and salt and process for 5 to 7 seconds, just until combined, scraping down the work bowl as necessary. Add the walnut halves and pulse about 10 times to incorporate them into the mixture and to chop them slightly.

Scrape the batter into the prepared pan and smoother the top with a rubber spatula. Bake the brownies for 25 to 30 minutes or until a cake tester or wooden pick inserted 2 inches away from the center comes out with a few moist crumbs clinging to it. Do not overbake the brownies.

Cool the brownies completely in the pan set on a wire rack. Using the two ends of the foil as handles, lift the brownies out of the pan. Invert onto a cutting board and peel off the aluminum foil. Re-invert the brownies so they are top side up. Using a sharp knife, trim the sides of the brownies so that they are straight and smooth. Place the brownies on a wire rack and set the wire rack over a baking sheet.

Bittersweet Chocolate Glaze
2/3 cup heavy whipping cream
6 ounces bittersweet chocolate, finely chopped

Do not make the glaze until the brownies are cool. In a medium saucepan over medium-low heat, bring the cream to a gentle boil. Take the pan off the heat and add the chocolate. Let the mixture stand for 30 seconds to melt the chocolate. Using a wire whisk, stir until the mixture is smooth. Strain the glaze through a fine sieve into a small bowl to remove any air bubbles. Cool the glaze for 5 minutes, or until slightly thickened but still pourable.

Pour the glaze evenly over the brownies, allowing the glaze to drip down the sides onto the baking sheet. Spread the glaze with a small metal offset cake spatula, making sure that the top and sides of the brownies are completely covered.