In a small bowl, combine the granulated sugar, light brown sugar and the zest of 1 medium to large lemon. Gently massage the zest into the sugar to draw out the essential oils and unique flavor.

In a large bowl, whisk together all-purpose flour, baking powder, cornstarch, and kosher salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, add the softened butter, sugar mixture and mix on medium-low speed until a smooth paste forms. Scrape down the sides and the paddle, then briefly mix again.

Add the egg and mix on medium until lightened.

Reduce the mixer to low and drizzle in the lemon juice. The batter will curdle slightly, but don’t stress about it.

Add about a third of the dry ingredients, then mix on low until flour has almost incorporated. Stop the mixer, add another third, and repeat until all the flour has been added.

Add the blueberries and gently fold them in to incorporate with a rubber or silicone spatula.

Scoop up about 2 tablespoons of the dough using your fingers or a spoon (each of my cookies weighed between 52-60g), and roll into a ball. Place on a parchment-lined baking tray. Press a few extra blueberries into the tops of the cookies for a nice looking finish. Wrap well in plastic wrap and refrigerate for at least 45 minutes or overnight.

Place cookies 2 inches apart on a baking sheet lined with parchment paper. Don’t bake on the same cold sheet pan from the fridge—it will alter bake time.

Bake for 13 minutes, regardless of whether the dough was chilled for 45 minutes or overnight. At 13 minutes, they were fully baked and had a nice crunch to the bottom but were soft and chewy in the center. When broken open immediately, they looked slightly under-baked, but after cooling on a wire rack, they turned out to be perfect.