How to cook
Knead a soft dough from the butter, egg yolk, powdered sugar, vanilla sugar and flour. Make small balls 1 oz (25 g) each. Roll them in the blended walnuts.
Arrange them in a tray laid out with baking paper. Make an indentation or press the middle of each ball (you have to feel the tray underneath but don't make a hole).
Bake 15 min. at 356°F (180 °C).
While you're waiting, put the jam in a heat-resistant bowl on the stove (jam needs to become runny). Fill the holes of the treats carefully with the jam using a spoon.