Preheat oven to 350 degrees F. Grease a 15x10x1-inch baking pan; set aside. In a large saucepan, combine 1/4 cup water and 2 tablespoons instant espresso powder or coffee crystals. Add the 1 cup butter and the chocolate pieces. Cook and stir over low heat until chocolate and butter are melted. Remove from heat. Add granulated sugar, eggs, and 1 teaspoon vanilla. Using a wooden spoon, lightly beat the mixture just until combined. Stir in flour; walnuts, if desired; and salt. Spread in prepared pan.

Bake in preheated oven for 20 to 25 minutes or until a wooden toothpick inserted near the center comes out clean. Cool in pan on a wire rack.

Meanwhile, for the frosting, place powdered sugar in a large mixing bowl. Add the 1/4 cup butter, 2 tablespoons boiling water, the coffee liqueur or 2 teaspoons instant espresso powder or coffee crystals, and the 1/2 teaspoon vanilla. Beat with an electric mixer on low speed until combined. Beat for 1 minute on medium speed. Spread frosting evenly over cooled brownies. If desired, sprinkle with coarsely chopped chocolate-covered coffee beans and additional espresso powder. Cut into bars. Makes 48 brownies.

Place bars in a single layer in an airtight container; cover. Store at room temperature for up to 3 days. Or cover pan of uncut, unfrosted bars with heavy foil and freeze for up to 3 months; thaw and frost.