Infuse sugars: In large bowl, combine brown sugar, granulated sugar, and lemon zest. Rub with fingertips for 1 minute until fragrant.
Cream butter: Cut cold butter into 1cm cubes. Add to lemon sugar. Beat on medium-high 4-5 minutes until mixture transforms from sandy to clumpy to light and fluffy.
Add egg: Add cold egg and lemon extract. Beat 2 minutes on medium until smooth and creamy.
Add dry: In separate bowl, whisk flour, cornstarch, baking soda, and salt. Add to wet mixture. Mix on lowest speed just until flour disappears.
Fold chips: Add white chocolate chips. Fold gently with spatula until evenly distributed.
Portion and shape: Divide dough into 5 portions of 170g each. Roll each into tall cylinder (hockey puck shape), not sphere. Place on parchment-lined tray.
Freeze: Cover tray with plastic wrap. Freeze minimum 8 hours (12-24 hours better).
Preheat and prep: When ready to bake, preheat oven to 190°C (375°F). Line baking sheet with parchment.
Bake: Place frozen dough cylinders on prepared sheet, 10cm apart. Bake 15-17 minutes until edges deep golden brown, centers still pale and soft.
Finish: Remove from oven. Immediately sprinkle with flaky sea salt. Rest on hot baking sheet 10 minutes.
