PREHEAT AND PREPARE: Five or ten minutes before you bake the cookies, preheat your oven to 350ºF. Line a baking sheet with parchment paper or a silicone baking mat.
COMBINE DRY INGREDIENTS: In a medium bowl, whisk together the all-purpose flour, baking soda, cornstarch, and salt. Set aside.
CREAM BUTTER AND SUGAR: In a large bowl, using an electric mixer, cream together the sugar and butter until light and fluffy. This should take about 2-3 minutes.
ADD WET INGREDIENTS: Beat in the egg, lemon juice, lemon zest, and vanilla extract, mixing until just combined.
INCORPORATE DRY INGREDIENTS: Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix.
FOLD IN BLUEBERRIES: Gently fold in the fresh blueberries, being careful not to crush them.
CHILL DOUGH: Cover the dough and chill in the freezer for at least 30 minutes or in the fridge overnight. This helps to prevent the cookies from spreading too much while baking.
SCOOP AND ARRANGE: Using a cookie scoop or a spoon, scoop the dough onto the prepared baking sheet, spacing them about 2 inches apart. Press a few additional fresh blueberries onto the top of each dough mound for a burst of flavor and color.
BAKE: Bake in the preheated oven for 15-18 minutes, or until the edges are golden brown and the centers are set but still a bit soft.
COOL: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.