Preheat oven to 325°F

Beat butter and sugar together until light and fluffy.

Add egg yolk and mix thoroughly.

Knead in the flour.

Roll into balls, flatten out on a baking sheet.

Make an impression in the center with the handle of a wooden spoon.

Bake for 10 -12 minutes.

Transfer to a wire rack.

When the cookies have cooled completely, fill a pastry bag with jam and pipe it into the middle.