In a bowl, cream together the butter and sugar. Add the rest of the ingredients and mix until blended. Wrap the dough in plastic and refrigerate until firm, at least 3 hours.
Preheat the oven to 350°. Roll out the dough to a ½-inch thickness and place on a baking sheet. Using a knife, score the dough into 24 bars. Bake until golden-brown, about 25 minutes. While the shortbread is warm, cut through the score marks with a sharp knife. Serve when cool.