Make the blueberry mixture. Combine blueberries and granulated sugar in a saucepan. Cook over medium heat for 5-7 minutes until reduced by half, stirring frequently. Stir in lemon juice and make a slurry with 1 tbsp of blueberry juices and Cornstarch. Pour into the saucepan, stir, and boil for 10 seconds. Transfer to a bowl and let cool completely.

Prepare the cookie dough base. In a bowl, combine softened butter, light brown sugar, remaining granulated sugar, lemon zest, and vanilla extract. Mix with an electric mixer on medium speed for 1-2 minutes until pale and fluffy. Mix in the egg until well combined.

Add dry ingredients. In a separate bowl, whisk together flour, baking soda, and salt. Add dry ingredients to the butter mixture and mix on low speed until just combined.

Swirl in the blueberry mixture. Divide dough in half into two bowls. Spread each half thinly. Dollop 1-2 tablespoons of the blueberry mixture over the dough. Use a knife to create swirls without fully incorporating.

Chill dough and preheat oven. Place dough in the refrigerator while preheating the oven to 350°F (175°C). Line baking sheets with parchment paper.

Scoop and bake. Use a cookie scoop to drop mounds of dough onto baking sheets, spacing them 3 inches apart. Optionally, place extra blueberries on top.

Cool cookies. Bake for 14-16 minutes until edges are browned. Let cool on baking sheets for 2 minutes before transferring to a wire rack.