Cream the softened butter and powdered sugar together until light and fluffy, about 3 minutes. Beat in the egg yolk and vanilla.

Mix in the flour and salt until a soft dough forms. Wrap in plastic wrap and chill for at least 1 hour. Clara chills hers overnight so she can make and bake them in one afternoon.

Preheat oven to 350°F and line baking sheets with parchment paper.

Roll the dough into 1-inch balls and place on the baking sheets about 2 inches apart. Press your thumb firmly into the center of each ball to create a well.

Fill each thumbprint with about 1/2 teaspoon of jam. Don't overfill or it will overflow during baking.

Bake for 10-12 minutes until the edges are just barely golden. The cookies will look slightly underdone, but they'll set as they cool.

Let cool on the baking sheet for 5 minutes before transferring to a wire rack. Clara wraps hers in clear cellophane with red ribbon for gifts.