Preheat your oven to 350°F. Line a 9×9-inch pan with parchment paper and set it aside.

In a large bowl, combine the melted butter and brown sugar. Stir until well mixed.

Add the eggs and egg yolk one at a time, stirring thoroughly after each addition.

Mix in the lemon juice and lemon zest.

In a separate medium bowl, whisk together the flour, cornstarch, baking powder, and salt.

Gradually stir the dry ingredients into the wet mixture until fully incorporated.

Gently fold in the blueberries.

Spread the batter evenly in the prepared pan. Bake for 43–48 minutes, or until the edges are golden brown.

Remove from the oven and let the blondies cool completely before adding the glaze.

In a small bowl, whisk together the confectioners' sugar and 2 tablespoons of lemon juice until smooth.

Add more lemon juice as needed to reach your desired consistency.

Drizzle the glaze over the cooled blondies, then cut into squares and serve.