Cream butter and sugar until light and fluffy. Add the beaten egg. Sift flour and salt together. Add flour to the creamed mixture and beat until just combined. Add lemon extract and the zest of 1 lemon to the mixture and combine well. Dough may be rolled, cut, or made into small balls for baking. Bake in a 375° oven until firm and golden. Remove from oven and place onto cooling rack and cool completely.