Preheat your oven to 375°F (190°C) and prepare a baking sheet with parchment paper.

Melt the unsalted butter in a microwave-safe bowl, then let it cool slightly.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt.

In a large mixing bowl, combine granulated sugar and lemon zest until it resembles wet sand, then add melted butter, egg, egg yolk, and vanilla extract.

Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.

Gently fold in fresh blueberries using a spatula, being careful not to mash them.

Using a medium cookie scoop, place balls of dough on the prepared baking sheet.

Bake for 10-12 minutes until edges are lightly golden and centers are soft.

Let cookies cool on the baking sheet for a few minutes before transferring to a wire rack.