In a medium saucepan over medium-low heat, combine fresh blueberries, granulated sugar, lemon juice, vanilla, and cornstarch. Cook for 30-40 minutes, stirring occasionally, until the mixture thickens to about 1/2 cup, and the blueberries burst and release their juices.

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

Using a hand mixer, beat the softened unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes. Add in the egg and vanilla extract, and mix until pale and creamy.

Slowly add the dry ingredient mixture to the creamed butter mixture, gently mixing until just combined. Fold in the cooled blueberry jam.

Line a baking sheet with parchment paper. Using a 2 tablespoon cookie scoop, portion out the dough and place on the baking sheet, leaving space for spreading.

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