1 cup (2 sticks) plus 2 tablespoons unsalted butter at room temperature
1/2 cup confectioner's sugar
1/2 teaspoon salt
1 tablespoon rum (optional)
2 cups all-purpose flour
2 ounces good semi-sweet chocolate

1. In the bowl of an electric mixer beat together 1 cup butter, the confectioner's sugar and salt until light and fluffy. (medium speed) Add the rum (or limoncello). Slowly, by increments, add the flour and beat until it is smooth and all of the flour is blended.

2. Place in an airtight container and chill for at least one hour, or as long as overnight.

3. Preheat the oven to 350 degrees F. (108 degrees C)

4. Using a measuring spoon, scoop up a teaspoonful of the dough and roll into a ball. Place the balls about one inch apart on an ungreased cookie sheet. Return any unused dough to the refrigerator.

5. Bake until firm but not brown. About 10 - 12 minutes. Transfer to a wire rack and allow to cool completely.

6. In the bottom of a double boiler, simmer just enough water to touch the upper pot. Add the chocolate and the remaining butter. Heat until the chocolate softens. With a whisk stir the chocolate until smooth. You want the consistency to be that of cake icing, slightly thick, not runny.

7. Using a butter knife or small offset spatula, spread a small amount of the chocolate on the bottom of one of the cookies. Add the top, mashing slightly. Place on a wire rack a allow to cool completely. Continue with the remaining cookies. Store in the refrigerator until time to serve. Can be kept for up to one week, refrigerated in an airtight container.