Step 1: Preheat your oven to 350°F (175°C) and line a large baking sheet with parchment paper or a silicone baking mat.
Step 2: In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until the mixture is light and fluffy.
Step 3: Add the egg, vanilla extract, and fresh lemon zest to the butter mixture and beat until well combined.
Step 4: In a separate bowl, whisk together the flour, baking soda, salt, and cornstarch.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Gently fold in the fresh blueberries using a spatula, being careful not to burst them.
Step 7: Scoop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Step 8: Chill the dough on the tray in the refrigerator for at least 30 minutes to prevent spreading.
Step 9: Bake for 10-12 minutes, or until the edges are lightly golden brown. The centers may still look slightly soft.
Step 10: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.