Prep the basics: Heat oven to 350°F (175°C).
Line two baking sheets with parchment paper. Zest and juice your lemon before you start.
Mix wet ingredients: In a large bowl, whisk melted, cooled butter with sugar until glossy, about 30 seconds. Whisk in egg, vanilla, lemon zest, and lemon juice until smooth.
Combine dry ingredients: In a separate bowl, whisk flour, cornstarch, baking soda, baking powder, and salt.
Make the dough: Add dry ingredients to the wet mixture.
Stir with a spatula until just combined. Fold in white chocolate chips. The dough should be soft but not sticky.
Rest briefly: Let the dough sit for 10 minutes. This quick rest helps prevent excessive spreading.
Portion: Scoop 1 1/2 tablespoon mounds (about 30–35 g each) onto prepared sheets, spacing 2 inches apart.
If you like, press a few extra chips on top and sprinkle with coarse sugar.
Bake: Bake 9–11 minutes, until edges are set and lightly golden but centers look slightly underdone. Do not overbake.
Set and cool: Let cookies rest on the sheet for 5 minutes to finish setting, then transfer to a rack to cool completely.