1. Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. Melt butter in a small bowl and cool in the fridge for about 10 minutes.
3. In another bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
4. In a large mixing bowl, whisk granulated sugar and lemon zest until the mixture resembles wet sand. Add the cooled melted butter, then whisk in the egg, egg yolk, and vanilla extract until smooth.
5. Stir dry ingredients into wet ingredients just until combined. Gently fold in blueberries, being careful not to overmix to avoid a cake-like texture.
6. Using a medium cookie scoop (about 2 tablespoons), form dough into balls. If desired, roll dough balls in granulated sugar for a classic sugar cookie finish before placing them on the baking sheet 3 inches apart.
7. Bake for 10-12 minutes, or until cookie edges are set and lightly golden. For frozen blueberries, reduce oven temperature to 350°F (177°C) and bake for 14-16 minutes.
8. Cool cookies on the baking sheet for 3-4 minutes, then transfer to a wire rack to cool completely.
9. (Optional) Whisk together lemon juice and confectioners’ sugar until smooth and drizzle over cooled cookies. Allow glaze to set for 15-20 minutes before serving.