In a medium pot, combine fresh blueberries, 1/4 cup of granulated sugar, 1 tablespoon of lemon juice, 1 teaspoon of vanilla extract, and 1/2 tablespoon of cornstarch. Cook on medium-low heat, stirring and gently mashing the blueberries as they soften. Within 30-40 minutes, the mixture will thicken to a jam-like consistency. Once done, remove from heat and allow the blueberry jam to cool completely.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of salt. This will ensure that your flour mixture is well combined and that the baking powder is evenly distributed. Set this bowl aside while you focus on the wet ingredients for your chewy blueberry cookies.

In a separate bowl, beat 1 stick of softened unsalted butter with 1 1/4 cups of granulated sugar using an electric mixer on medium speed. Continue mixing for about 2 minutes until the mixture is light and fluffy. Next, add 1 egg and another teaspoon of vanilla extract, and mix for 1-2 minutes until fully incorporated and pale in color, creating the base for your cookie dough.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; the dough should be soft and slightly sticky. Once combined, gently fold in the cooled blueberry jam using a spatula, ensuring that the jam is evenly distributed throughout the dough without overworking it.

Using a large cookie scoop, approximately 2 tablespoons in size, portion the cookie dough onto a lined baking sheet. Space them about 2 inches apart to allow for spreading. To add a touch of sparkle, sprinkle a little granulated sugar on each cookie mound before baking.

Preheat your oven to 350°F (175°C). Bake the cookies for 10-13 minutes, or until the edges are set and the tops appear slightly puffy. Let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to finish cooling completely.