Begin by preheating your oven to 375°F about 20-30 minutes before you plan to bake the cookies. Line a large baking sheet with parchment paper and set it aside.
In a small bowl, melt the butter and place it in the fridge to cool for approximately 10 minutes.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt until they are well combined. Set this mixture aside.
In a large mixing bowl, combine the granulated sugar and lemon zest. Whisk them together until the mixture resembles wet sand. Add the cooled melted butter to the sugar-zest mixture and whisk until well combined. Then, whisk in the whole egg, egg yolk, and vanilla extract until the mixture is smooth and well incorporated.
Gradually stir the dry ingredients into the wet ingredients until just combined. Be careful not to overmix. Gently fold in the fresh or frozen blueberries until they are evenly distributed throughout the dough.
Using a medium cookie scoop (2 tablespoons), portion the lemon blueberry cookie dough into balls. If you'd like to roll the cookies in sugar, do so now and place them on the prepared baking sheet, leaving about 3 inches of space between each cookie.
Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are set and light golden brown.
Allow the chewy lemon blueberry cookies to cool on the baking sheet for 3-4 minutes before transferring them to a cooling rack. Let them cool completely to room temperature before serving and enjoying your delightful creation!