Preheat oven to 300F/150C and line a large baking sheet.
Pulse the walnuts in a food processor until they're coarsely ground.
Whisk together the egg yolks, sugar and vanilla for a few minutes until the mixture becomes paler and glossy.
Fold in the ground walnuts and stir until well combined, but try not to over-stir so you still have a bit of air in the mixture.
Take small spoonfuls (heaped teaspoonfuls) of the mixture and put on a baking sheet, with around 1in/2.5cm or more between them. Smooth off the pieces of dough so they are fairly round and gently press a piece of walnut into the top of each cookie.
Bake for approximately 15 minutes until lightly brown. Leave them to cool on the baking sheet a few minutes as they will be a little soft.
Video
Notes
If you like, you can add some cardamon and/or rosewater for an additional fragrant flavor - I'd recommend no more than about ¼ teaspoon for one recipe quantity, then adjust from there if you want a stronger flavor.