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.