In a medium saucepan over medium-low heat, combine fresh blueberries, granulated sugar, lemon juice, vanilla extract, and cornstarch. Stir gently for 30–40 minutes, allowing the mixture to bubble and thicken until it reaches a jam-like consistency. Set aside to cool.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

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

In a large bowl, beat the softened unsalted butter and granulated sugar until light and fluffy.

Add the egg and an additional splash of vanilla extract to the butter-sugar mixture. Beat until pale and fluffy.

Gradually mix the dry ingredients into the wet ingredients, stirring just until a dough forms. Gently fold in the cooled blueberry jam.

Using a cookie scoop, drop portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-13 minutes until the edges turn golden brown but the centers remain soft.

Allow to cool on baking sheets for about 5 minutes before transferring to a wire rack.