Preheat the oven to 375°F (190°C) and line a large baking sheet with parchment paper.

Melt the butter and let it cool in the fridge for about 10 minutes.

In a bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.

In a large mixing bowl, whisk the sugar and lemon zest until it resembles wet sand. Whisk in the cooled butter, then the egg, egg yolk, and vanilla extract until smooth.

Combine the dry ingredients with the wet, then gently stir in the blueberries.

Using a medium cookie scoop, form dough balls. Optionally roll them in sugar and place on the baking sheet.

Bake for 10-12 minutes or until lightly golden. Cool on the sheet before transferring to a rack.