Preheat & Prep: Preheat the oven to 350 °F (175 °C) and line baking sheets with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together flour, cornstarch, baking soda, and salt. Then, set it aside.
Cream Butter & Sugar: In the bowl of a stand mixer (or with a hand mixer), mix cream butter and granulated sugar until they are light and fluffy, for about 2–3 minutes.
Add Wet Ingredients: Add the egg, lemon zest, lemon juice, and vanilla extract. Make sure you mix them until they are fully combined.
Combine Dry + Wet: Slowly add the dry ingredients to the wet, mixing on low until just combined. But avoid over-mixing
Add Mix-Ins: Stir in the white chocolate chips until evenly distributed.
Scoop Dough: Using a ¼-cup scoop (or dough scoop), place rounded balls of dough about 2 inches apart on prepared baking sheets. L
Bake: Bake at 350 °F for 12–15 minutes, until the edges are just beginning to turn golden.
Cool & Garnish: Let cookies cool on the baking sheet for 5 minutes, transfer to a rack, then dust lightly with powdered sugar once fully cooled.