In a stand mixer fitted with the paddle attachment, beat butter, sugar, and lemon zest for 3 to 4 minutes until light and fluffy. Scrape down the sides as needed.
Add the egg, egg yolk, and lemon extract. Mix until combined.
Add flour, baking soda, cornstarch, and salt to the bowl.
Pour lemon juice directly over the baking soda to activate it.
Mix just until combined. Do not overmix the dough.
Scoop dough into balls about 1½ to 2 tablespoons each, or refrigerate the dough before scooping.
Chill dough for at least 1 hour.
Preheat oven to 325°F.
Roll cookie dough balls in granulated sugar.
Arrange 6 to 8 cookies per baking sheet lined with parchment paper or silicone baking mats.
Bake for 10 to 12 minutes. Do not overbake; the cookies should remain soft and chewy.
Immediately swirl a large cup around the warm cookies to shape them into perfect circles.
Let cookies rest on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Garnish with extra sugar and fresh lemon zest if desired.