Preheat your oven to 375°F (190°C).

In a medium bowl, combine the fresh blueberries, granulated sugar, cornstarch, lemon juice, vanilla extract, and salt. Gently toss the mixture until the blueberries are evenly coated. Set aside.

In a large bowl, mix together the flour, softened butter, powdered sugar, egg, and baking powder. Use a pastry cutter or your fingertips to combine until the mixture resembles coarse crumbs.

Take about two-thirds of the dough and press it evenly into the bottom of a greased 9×9-inch baking dish to form the base.

Pour the blueberry mixture over the crust, spreading it evenly.

Roll out the remaining dough between two pieces of parchment paper to about 1/8 inch thick. Cut into strips and create a lattice design over the blueberry filling.

Bake in the preheated oven for 30-35 minutes or until the crust is golden brown and the blueberries are bubbling.

Let the bars cool for about 10 minutes before slicing into squares.