In a medium pot, combine fresh blueberries, granulated white sugar, lemon juice, vanilla extract, and cornstarch. Cook over medium-low heat for 30 to 40 minutes, stirring occasionally, until the mixture thickens. Mash the blueberries halfway through cooking. Once thickened, remove from heat and allow it to cool completely.

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

In a large bowl, beat softened unsalted butter and granulated sugar using an electric mixer on medium speed until fluffy, about 2 minutes. Add in the egg and vanilla extract, mixing for another 1 to 2 minutes until the mixture turns pale and smooth.

Gradually incorporate the dry ingredient mixture into the creamed butter and sugar. Mix on low speed just until a dough forms. Gently fold in the cooled blueberry jam with a spatula.

Using a 2-tablespoon cookie scoop, portion the dough onto a lined baking sheet, spacing them about 2 inches apart. Sprinkle the tops lightly with granulated sugar.

Preheat your oven to 350°F (175°C). Bake the cookies for 10 to 13 minutes, or until the edges are lightly golden and the centers are set. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack.