In a medium pot, combine fresh blueberries, granulated white sugar, lemon juice, vanilla, and cornstarch. Cook over medium-low heat for 30-40 minutes, mashing blueberries until thickens to a jammy consistency. Let cool completely.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. In a large bowl, whisk together the all-purpose flour, baking powder, and salt.

In another bowl, cream the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Then add the egg and vanilla, mixing until well combined.

Gradually add the dry flour mixture to the wet ingredients, stirring until a soft dough forms. Fold in the cooled blueberry jam, ensuring even distribution.

Using a cookie scoop or two spoons, portion out the dough onto the prepared baking sheets, spacing them about 2 inches apart. Sprinkle sugar over the top if desired.

Bake for 10-13 minutes, until edges are lightly golden and tops look set. Let cool on baking sheets for 5 minutes before transferring to a wire rack.