Place the flour, baking soda, cornstarch and salt(if using) in a medium bowl and stir. Set aside.
Put the lemon zest into a mixing bowl and add the sugar. Mix them with a fork or rub them with your fingertips or use the paddle attachment to stir if using a stand mixer to bring the lemon flavor out.
Beat the butter and sugar mixture on medium speed until the mixture is creamy with a hand-held mixer or with a paddle attachment if using a stand mixer. During these steps, stop the machine and scrape down the sides and bottom of the bowl with a rubber spatula as needed to make sure that they are all mixed.
Mix in the egg. Stop the machine and scrape down the sides and bottom of the bowl with a spatula as needed.
Add the lemon juice and stir until mixed. During this stage, the mixture will curdle which is totally fine.
Stir in the flour mixture in 3 batches.
Fold the blueberries with a spatula in 3 to 4 batches. This makes the folding step easier.
Cover the dough with a plastic film and keep it in the freezer for 30 minutes until firm. When you make balls, the dough shouldn't stick to your hands.
Preheat the oven to 325°F(165C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Remove the dough from the freezer and make 2 tablespoons of big balls (40 grams) out of the cookie dough. Place the cookie dough balls onto the baking sheet 2 inches (5cm) apart and bake for 11-13 minutes or until the top of the cookies are slightly set, the edges are slightly brown and the cookies are soft to handle. The cookies will be puffy when they are out of the oven but they will deflate slightly after a few minutes. They will set as they cool on the baking sheet.