In a medium saucepan, combine fresh blueberries, granulated sugar, lemon juice, and vanilla extract over medium-low heat. Cook while gently stirring for 30-40 minutes until the mixture thickens and resembles a jam consistency.

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

Using a hand or stand mixer, beat the softened unsalted butter and granulated sugar together in a separate bowl for about 2 minutes until light and fluffy.

Add the room-temperature egg and a splash of pure vanilla extract to the creamed butter and sugar mixture. Beat until pale and fluffy, about 1-2 minutes.

Gradually add the whisked dry ingredients to the wet mixture, mixing just until combined. Gently fold in the cooled blueberry jam.

Using a cookie scoop, portion out the dough on a parchment-lined baking sheet, spacing the dough balls about 2 inches apart.

Preheat your oven to 350°F (175°C) and sprinkle granulated sugar on top of each dough ball. Bake for 10-13 minutes or until the edges are set.