Preheat oven to 300 degrees F.

In a bowl, beat the egg yolks, sugar and vanilla until mixture has thickened and turned a pale yellow, about 8-9 minutes.

Add the ground walnuts and mix well.

Cover 3 baking sheets with parchment paper or greased aluminum foil.

Using a teaspoon, drop small portions of the walnut mixture on the sheet; make sure to leave enough space between them.

Bake on the middle rack of the oven for 14-16 minutes or until golden, keep an eye on them (they look soft but it is ok, they start to harden when they cool).

Remove from the oven and let cool on the baking sheet for 2-3 minutes. Then transfer the cookies on a wire rack to cool completely. Store them covered.