Several hours or the day before baking: In a medium bowl, sift together the flour, baking powder, and salt. With an electric mixer, beat the butter and both sugars until creamy and well blended, about 5 minutes. Beat in the egg, scrape down the sides of the bowl, and beat again until blended. Add the flour mixture, beating just until combined, and then mix in the nuts.

Transfer the soft dough to the center of a 12-inch-long piece of plastic wrap. Cover with the wrap and shape the dough into an 8-inch-long squared-off log. (Since the dough is soft, it may be hard to square off the sides completely; after chilling, the dough is easier to model.) Refrigerate for at least 3 hours or overnight.

To bake: Heat the oven to 350 degrees F. Cover your cookie sheets with parchment. Cut the firm dough crosswise into 1/4-inch-thick slices. Arrange the slices about an inch apart on the sheets. Bake the cookies, one sheet at a time, on a middle rack until the edges just begin to brown, 12 to 14 minutes. Let cool on the cookie sheet for 5 minutes; transfer to a wire rack to cool completely.