Preheat your oven to 350°F and arrange your oven rack to the middle position. Line 2 large baking sheets with parchment paper or silicone baking mats.
Whisk the flour, salt, baking soda, baking powder, and cornstarch in a medium bowl.
In a large bowl, use an electric mixer to beat the softened butter until smooth. Add the granulated sugar and mix on medium speed for 2 minutes until light and fluffy.
Add the egg and extra yolk and mix for 2 minutes until well combined. Mix in the fresh-squeezed lemon juice, then add the dry ingredients.
Mix until the cookie dough comes together, then fold in the blueberries and white chocolate chips.
Scoop the dough into 2-4 tablespoon-size balls, depending on how big you want the cookies to be. Toss the dough in sugar or sparkling sugar, if desired.
Arrange the cookie dough on the lined baking sheet, about 2 inches apart. Bake, one sheet at a time, for 10-12 minutes. The cookies should be just starting to brown around the edges.