In a medium-sized pot, combine fresh blueberries, granulated sugar, lemon juice, vanilla extract, and cornstarch. Cook over medium-low heat for 30-40 minutes, stirring occasionally, until thickened. Remove from heat and let cool.

In a large mixing bowl, whisk together all-purpose flour, baking powder, and salt until well combined. Set aside.

In a separate bowl, cream the softened unsalted butter and granulated sugar until light and fluffy. Add the egg and vanilla extract, mixing until pale.

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

Gently fold in the cooled blueberry jam until you see streaks of purple.

Portion the dough onto a parchment-lined baking sheet, spacing them 2 inches apart. Sprinkle with granulated sugar if desired.

Preheat your oven to 350°F (175°C). Bake for 10-13 minutes until edges are golden and centers are soft. Cool on the pan for 5 minutes before transferring to a wire rack.