INSTRUCTIONS :
With a mixer or hand mixer, beat the butter and sugar on medium speed until light and fluffy, about 2 minutes.
Add the vanilla and beat on medium speed until blended. Gradually add the flour mixture and beat on low speed until combined. Add the walnuts and continue beating until smooth.
Turn the mixer to high speed and beat for another 2 minutes, until the mixture resembles breadcrumbs and is less dry (the mixture should easily form a ball with your hands).
Line two translucent baking trays with baking paper or a silicone mat. Take a spoonful of dough, form it into a 2.5 cm ball with your hands and place it 2.5 cm apart on the baking tray. When the baking tray is full, place it in the refrigerator and leave it to cool for an hour.
Preheat the oven to 350 F. Bake the cookies for 15-18 minutes, until golden brown on the bottom edges.
Allow the cookies to cool on the baking tray for 5 minutes, then, while still warm, sprinkle them with icing sugar in a bowl until completely coated. Transfer the cookies to a wire rack to cool completely. By this time, the sugar will have melted and become slightly sticky. When the cookies have cooled completely, dip them again in the icing sugar.
