Festive Thumbprints

Measure the nuts into the bowl of a food processor. Pulse the machine to finely chop the nuts, then set them aside. Using an electric mixer, cream the butter. Gradually beat in the sugar until fluffy. Beat in the egg yolk and vanilla extract. Sift the flour, baking powder, and salt into a bowl. Then stir the flour mixture into the butter one third at a time. Stir in the chopped nuts. Cover the dough with plastic wrap and refrigerate it for 20 to 30 minutes. Lightly grease 2 large baking sheets. Heat the oven to 325°. Roll the dough into 1 1/4-inch-thick balls and place them on the sheets about 1 1/2 inches apart. Next, press your thumb into each ball to make a deep impression. (Tip: Before shaping the dough balls and pressing your thumb into the centers, dust your hands with flour to keep the dough from sticking.) Bake the cookies on the center oven rack for 20 minutes, then transfer them to a wire rack to cool completely. Spoon about 1/2 teaspoon of filling into each thumbprint. Store the cookies in a tin until serving time. Makes about 2 dozen cookies.