Our friend Mildred Kroll used to make this quick version of shortbread as a snack for her kids when they went off to college.
Position a rack in the center of the oven. Preheat the oven to 300°F. Have ready a 13 x 9-inch baking pan.
Beat on medium speed until very fluffy and well blended:
      1/2 pound unsalted butter, softened
      1/2 cup superfine sugar
Melt, stirring often, in the top of a double boiler or in a microwave on medium:
      2 ounces semisweet chocolate or bittersweet chocolate
Remove from the heat and let cool slightly. Sift over the top:
      2 cups all-purpose flour
Add the melted chocolate, then stir until well blended. Press the dough into the pan to form a smooth even layer. Bake until the top is firm when lightly pressed and a toothpick inserted in the center comes out clean, about 40 minutes. Remove the pan to a rack and let cool until barely warm. Cut into bars and transfer to a rack to cool.