Texas Sheet Pan Brownies

3 cups all-purpose flour1 1/2 cups light brown sugar, packed1 1/2 cups sugar1 cup unsalted butter (2 sticks) 1/2 cup butter flavored shortening1 1/2 cups strong brewed coffee, cooled 1/2 cup unsweetened cocoa powder 3/4 cup buttermilk (see note)3 large eggs1 1/2 teaspoons baking soda2 teaspoons vanilla extractChocolate glaze 1/2 cup unsalted butter (1 stick)3 tablespoons unsweetened cocoa powder 1/3 cup milk3 1/2 cups powdered sugar1 teaspoon vanilla extractChopped walnuts or pecans, optional garnishHeat oven to 350 degrees. Lightly grease bottom of heavy half-sheet pan, 12 by 18 inches.For the brownies: In a large mixing bowl whisk together flour and the sugars. Set aside.In heavy saucepan, combine butter, shortening, coffee and cocoa. Stir constantly and heat until boiling. Cool 1 minute.Pour hot mixture over flour and sugar in the bowl. Add buttermilk, eggs, baking soda and vanilla. Stir by hand until combined. Pour into prepared pan. Smooth top with spatula. Bake until tester inserted at center comes out clean, about 40 to 45 minutes.While brownies bake, prepare the glaze.For the glaze: In heavy saucepan, combine the butter, cocoa and milk. Stir constantly and heat until just boiling. Mix in the powdered sugar and vanilla. Stir until glaze is a smooth consistency.Pour warm glaze over brownies immediately after baking. Quickly smooth with spatula. Top with chopped nuts, if desired. Cool 2 to 3 hours at room temperature before cutting into bars.Serves 48.Baker's hint: If you don't have buttermilk, add 1 teaspoon vinegar to 3/4 cup milk; set aside for 5 minutes.Nutrition values per serving: 184 calories, 9 g fat (4g saturated), 26 g carbohydrates, 1 g fiber, 2 g protein, 29 mg cholesterol, 51 mg sodium.