Cream butter and sugars together. Beat in egg and vanilla.
Mix in lemon zest
In a separate bowl combine flour, cornstarch, baking soda and salt. Add to butter mixture.
Stir until combined and gently stir in white chocolate chips.
Form into balls about an inch or slightly more in diameter.
Chill for at least 2 hrs or a couple days if you don't want to eat them right away.
Preheat oven to 350 and grease cookie sheet. Bake for 8-11 minutes until edges are lightly brown. Do not over-bake. The centers may still look doughy.
Allow to cool on the cookie sheet for 5 minutes and then cook on wire rack.