9 ounces Swiss dark chocolate, coarsely chopped
6 ounces unsweetened chocolate, coarsely chopped
12 tablespoons unsalted butter, softened
3/4 cup granulated sugar
3/4 cup packed dark brown sugar
3 large eggs (at room temperature)
1/3 cup Grand Marnier
1 teaspoon vanilla extract
1/2 teaspoon finely grated orange zest
1 1/4 cups all-purpose flour
1/4 teaspoon salt
1 1/2 cups coarsely broken walnuts
15 chocolate-dipped walnuts, for garnish (optional)

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

Melt chocolates. Cool the chocolate for 10 to 15 minutes, until tepid.

In a large bowl, using a hand-held electric mixer set at high speed, beat together the butter and sugars until creamy. One at a time, beat in the eggs, beating well after each addition. Beat in the Grand Marnier, vanilla extract and orange zest until combined. The mixture may look slightly curdled. Beat in the cooled chocolate until smooth. On low speed, beat in the flour and salt just until mixed. Using a rubber spatula, stir in the broken nuts.

Scrape the batter into the prepared pan and smooth the surface 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.

Cool the brownies in the pan on a wire rack. Place the brownies in the pan on a wire rack. Place the brownies in the pan in the freezer and chill for 20 minutes. Using the two long ends of the foil as handles, lift the brownies out of the pan. Gently invert the brownies onto a cutting board or large plate and carefully peel off the foil. Invert again onto a smooth surface and cut into 15 squares. Top with chocolate-dipped walnuts, if desired.