In a medium bowl sift together flour, cornstarch, baking soda and salt. Set aside.
In a medium bowl, mix together well granulated sugar, brown sugar and lemon zest. Set aside.
In a mixer bowl fitted with the paddle attachment, beat the butter on medium speed for one minute until creamy. Add sugar mixture and beat on medium speed for about 3 minutes, until light and fluffy. Beat in the egg. Add flour mixture and mix just until combined. Do not overmix. Fold in white chocolate chips.
I highly recommend (i.e. this is not mandatory and this step can be skipped) chilling the dough for at least 1 hour (or up to 3 days). This step results in thicker cookies that won’t spread in the oven while baking.
Preheat oven to 350F/180C degrees. Line pans with parchment paper.
If the cookie dough is hard to handle after refrigerating, leave it on the counter for a few minutes.
Drop rounded balls of dough onto prepared baking sheet, the size of 1.5 tablespoons each (use an ice-cream scoop or heaping tablespoon). Bake for about 8 minutes, until cookies just begin to brown at the edges, but the center is still soft. Cookies will appear undone but will continue to bake on the warm baking sheet. Leave to cool for 10 minutes or until the cookies are easy to transfer without breaking (that’s how soft they are!). Then gently transfer to a wire rack to cool completely.
Store cookies at room temperature in an airtight container for up to 3 days.
Cookies can be frozen for up to 2 months.