Whisk together the flour, baking powder, cornstarch and salt. Set aside.
In the bowl of a stand up mixer or with a hand held mixer, cream together the butter and sugar until light and creamy (approximately 2-3 minutes).
Scrape down the sides of the bowl and mix in the eggs, vanilla extract, lemon zest and lemon juice on low speed until combined. Then turn the speed up to medium and mix for a few minutes.
Change the speed back down to low on the mixer and gradually mix in the dry ingredients and the milk until just combined. Be careful not to over mix the batter.
Carefully hand stir in the blueberries with a spoon or spatula.
Cover and refrigerate the dough for at least 20-30 minutes.
Preheat the oven to 350 degrees F and line 2 baking sheets with parchment paper.
Use a 1 1/2 tablespoons cookie scoop to scoop balls of dough and place them on the prepared baking sheets at least 2 inches apart.
Bake for 12-15 minutes until the edges of the cookies are slightly golden brown.
Remove the cookies from the oven and let them sit for 5 minutes and then transfer them to a wire rack to cool completely.