In a medium saucepan, combine fresh blueberries, granulated sugar, lemon juice, vanilla, and cornstarch. Cook over medium heat while stirring for about 30-40 minutes, allowing the mixture to thicken and bubble gently. You’ll know it’s ready when it coats the back of a spoon. Remove from heat and let the jam cool while you mix the cookie dough.

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

Beat the softened unsalted butter and granulated sugar for about 2 minutes until light and fluffy.

Add the room-temperature egg and a splash of vanilla extract to the butter-sugar mixture, mixing on low speed until well incorporated.

Gradually add the dry ingredient mixture to the wet batter, mixing on low speed until a soft dough forms.

Gently incorporate the cooled blueberry jam into your cookie dough with a spatula, aiming for a marbled effect.

Using a 2-tablespoon cookie scoop, portion out the dough onto lined baking sheets, ensuring space between each mound.

If desired, sprinkle a touch of granulated sugar on top of each cookie mound. Bake at 350°F for 10-13 minutes, or until edges are golden brown.

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