In a medium pot, combine fresh blueberries, granulated sugar, lemon juice, vanilla extract, and cornstarch. Cook over medium-low heat for about 30–40 minutes, stirring frequently, until the mixture thickens and some blueberries are mashed.

In a large mixing bowl, whisk together all-purpose flour, baking powder, and salt until fully blended.

Using a hand mixer, beat the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.

Add the room temperature egg and pure vanilla extract to the butter and sugar mixture, mixing until pale and creamy, about 1-2 minutes.

Gradually mix the dry ingredients into the wet mixture, stirring until a dough forms.

Gently fold the cooled blueberry jam into the cookie dough until swirled.

Using a cookie scoop, portion the dough onto lined baking sheets, leaving about 2 inches of space between each scoop.

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