Preheat oven to 350 degrees F.In a medium mixing bowl, cream together the sugar and butter. Beat in the egg white and vanilla until smooth. Beat in the lemon zest and lemon juice.

Fold in the flour, cornstarch, and baking powder.

Take a teaspoonful of dough and roll it into a ball with your hands. Place on a baking sheet and flatten with the bottom of a glass. Repeat. Sprinkle extra sugar on top, if desired.

Bake for 8-12 minutes, or until the edges barely begin to brown. Transfer to a cooling rack and cool completely. Store in an airtight container at room temperature.