Cream butter and sugar until light. Beat in eggs, lemon zest and a bit of lemon extract. Stir in flour, baking soda, cream of tartar, and salt to form a soft dough. Chill dough briefly. Roll into balls and then roll in sugar. Place on baking sheet and flatten slightly. Bake at 350?F until just set (edges lightly golden), about 8?10 minutes..