Optional: 1 tbsp chopped cashews, or a little jam for the center
How to make it
Cream (3 min): Beat butter and powdered sugar until pale and fluffy. Beat in the yolk and vanilla or zest.
Flour (2 min): Mix in the flour and salt to a soft dough, adding the milk only if it's crumbly.
Chill (20 min): Pat flat, wrap and chill 20 minutes.
Cut (10 min): Roll to 7 mm (just over 1/4 in) thick, thicker than most cookies. Cut 6 cm fluted rounds and set on lined trays. Mark with a fork, or press a cashew or a dab of jam in the middle.
Bake (15 min): Bake at 160C (325F) for 15 to 18 minutes, until set with barely golden edges. Cool on the tray; they crisp as they cool.