Beat the egg whites until stiff enough to leave a knife cut visible when tested. Add the sifted powdered sugar and butter vanilla flavoring and continue beating over boiling water, removed from heat, until the mixture is glossy. Quarter the walnuts and gently fold into the egg white mixture.
Using two teaspoons, place small mounds on a baking sheet lined with parchment paper. Place the tray in the middle of the preheated oven and bake at 160°C – 180°C for approx. 8-10 min. so that the macaroons are only lightly browned on the outside and still ‘pull’ a little on the inside.