Preheat the oven to 375°F. Line two baking sheets with parchment paper or silicone baking mats and set aside.
Beat the butter in the bowl of a stand mixer fitted with the paddle attachment for 30 seconds on medium-high speed. Add in the sugar, lemon zest, cornstarch, baking powder, baking soda and salt. Scrape the sides of the bowl with a rubber spatula as needed.
1/2 cup unsalted butter, softened (8 Tbsp.), 1 cup granulated sugar, 1 Tbsp. lemon zest (or more for more lemony flavor), 1 tsp. cornstarch, 1/2 tsp. baking powder, 1/2 tsp. baking soda, 1/2 tsp. salt
Beat in the egg, sour cream, lemon juice, lemon and vanilla extract.
1 large egg, room temperature, 1/2 cup sour cream, room temperature, 1-2 Tbsp. lemon juice, 1 tsp. lemon extract, 1 tsp. vanilla extract
Turn the mixture down to medium speed and gradually add in both flours.
1 1/2 cups all-purpose flour, 1 1/2 cups cake flour
Add in the white chocolate chips just until combined.
3/4 cup white chocolate chips
Drop dough onto prepared baking sheet. I like to use a 3 Tbsp. cookie scoop for this because the batter can be very fluffy/sticky. (recipe will make ~12-14 cookies)
Bake in preheated oven for 11-13 minutes or until lightly browned on the sides. The centers will still look very soft*. (See tip below)
Make the optional lemon glaze while the cookies are in the oven. In a small mixing bowl, stir together the sugar and lemon juice.
1/4 cup granulated sugar, 2 Tbsp. lemon juice
Brush the tops of the warm cookies with just a little bit of the the lemon glaze. Allow cookies to cool on the baking sheet for 10-15 minutes before transferring to a wire cooling rack. Garnish with more lemon zest, if desired, and enjoy.