Preheat the oven to 440°F/225°C. Beat the butter with ¾ cup of the powdered sugar until thick and fluffy. Add the egg yolks one by one. Mix in the lemon zest and alcohol. Add the flour and cornstarch, and stir. Beat the egg whites with the salt and remaining powdered sugar until stiff. Stir about 1/3 of the egg whites into the yolk mixture, then stir in the flour mixture. Fold in the remaining egg whites.