Combine fresh blueberries, granulated sugar, lemon juice, pure vanilla extract, and cornstarch in a saucepan. Cook over medium-low heat for 30-40 minutes until the mixture thickens and resembles jam. Let it cool completely.

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

In a large bowl, beat softened unsalted butter and granulated sugar until fluffy and light, about 2 minutes.

Add the egg and pure vanilla extract to the butter mixture and blend until well incorporated.

Gradually add the whisked dry ingredients into the wet mixture. Mix gently until a dough forms.

Carefully fold the cooled blueberry jam into the cookie dough.

Using a cookie scoop or tablespoon, drop portions of the dough onto a lined baking sheet.

Bake the cookies in a preheated oven at 350°F (175°C) for 10-13 minutes or until lightly golden.

Allow them to cool on the baking sheet for 5 minutes before transferring to a cooling rack.