In a medium saucepan over medium-low heat, combine fresh blueberries, granulated sugar, lemon juice, vanilla extract, and cornstarch. Cook for 30–40 minutes, stirring occasionally, until thickened.

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

In a separate mixing bowl, beat softened unsalted butter and granulated sugar together until light and fluffy, about 2 minutes.

Add in egg and vanilla extract, mixing until pale and fluffy, about 1–2 minutes.

Gradually add dry ingredients to the wet mixture, mixing on low speed until just combined.

Gently fold in the cooled blueberry jam with a spatula.

Scoop dough onto lined baking sheets, spaced about 2 inches apart.

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