Beat the softened butter with the powdered sugar until whitened. Add the egg yolks, vanilla and flour. Knead a soft dough.
Form treats with a small teaspoon and arrange them on baking paper in an oven dish.
Put the oven dish in a preheated 356°F (180 °C) oven and bake until slightly reddened. Take them out. Once cooled, stick them together in pairs using a jam of your choice.
Next, dip one side of the treats in melted chocolate, roll them in finely ground walnuts and leave to harden.
They taste better after they've sat for a bit. Enjoy!