Cream butter and sugar until very light and fluffy.
Add egg yolk and beat in.
Fold in flour and baking powder.
Add lemon rind and vanilla.
Beat egg whites until stiff and fold in.
Bake in a greased and floured 11x16x2 inch pan. Sprinkle ground walnuts on top of batter. Bake at 350 degrees for 20-25 minutes. Cool.
Cut into half moons with a glass; sprinkle with powdered sugar.
Variations: You don’t have to use the ground walnuts. You can leave them off altogether or substitute with another nut. I imagine crushed almonds and almond extract would be a nice combination. Play around with the flavorings and find your favorite!