Whisk butter, sugar, and vanilla in a medium bowl until creamy.
225 g butter, at room temperature, 70 g sugar, a few drops of vanilla extract
Fold in the flour and the ground walnuts (you can toast them briefly in a pan for a nuttier flavor, or put them in the dough simply raw), stirring well.
250 g all-purpose flour, 160 g ground walnuts
Transfer the cookie dough to the fridge for about an hour, until firm; meanwhile, line a baking sheet with parchment paper, and preheat the oven to 350°F.
Shape the dough into small balls and place on the baking sheet. The cookies will spread a tiny bit during cooking, but not very much, so if you want them to be flat you should press them down to flatten them with the bottom of a glass. The ones in the picture were simply baked in the shape of a ball, without any additional handling.
Bake the cookies in the preheated oven until the bottom is golden, for 15/20 minutes. They will seem a bit crumbly just out of the oven, but if you leave them on the baking sheet to cool they will be perfect.