In a medium saucepan, combine fresh blueberries, granulated sugar, lemon juice, vanilla extract, and cornstarch over medium-low heat. Cook for 30-40 minutes until it thickens to jam consistency, mashing blueberries halfway through.

Remove from heat and allow the blueberry jam to cool completely.

In a mixing bowl, whisk together all-purpose flour, baking powder, and salt until evenly combined.

In a large mixing bowl, beat the softened unsalted butter and granulated sugar together on medium speed for about 2 minutes until light and fluffy.

Add in the room-temperature egg and a splash of vanilla extract, continuing to beat until pale and well combined.

Gradually add the dry mixture into the creamed butter and sugar, mixing on low speed just until combined.

Gently fold in the cooled blueberry jam into the dough, ensuring it's evenly distributed.

Using a 2-tablespoon cookie scoop, portion out the dough into balls and arrange on a parchment-lined baking sheet.

If the dough is too sticky, refrigerate it for about 30 minutes before shaping.

Sprinkle granulated sugar lightly over the tops of each cookie ball before baking.

Preheat your oven to 350°F (175°C) and bake for 10-13 minutes until edges are lightly golden.

Let cookies cool on the baking sheet for about 5 minutes before transferring to a cooling rack.