Step 1: Prepare the Cookie Dough

Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats for easy removal of the cookies and even baking.

In a medium-sized bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, and 1/4 teaspoon salt. This ensures that your dry ingredients are well-mixed and ready to rise.

In a separate, large bowl, cream together the 1/2 cup softened butter and 3/4 cup granulated sugar using an electric mixer. Beat on medium speed for 3-4 minutes until the mixture becomes light and fluffy.

Add the 1 large egg and 1 teaspoon vanilla extract to the butter-sugar mixture and continue to beat until fully incorporated. Stir in the 1/2 teaspoon lemon zest for a fresh citrus flavor.

Gradually alternate adding the dry ingredients and 1/2 cup sour cream into the wet mixture, starting with a third of the dry ingredients, followed by half of the sour cream. Continue mixing gently after each addition until the dough is smooth and creamy.

Gently fold in the 1 cup fresh blueberries, being careful not to break them up too much.

Step 2: Prepare the Jam

In a medium saucepan, combine the 1 cup blueberries, 1/2 cup sugar, and 2 tablespoons lemon juice. Cook this mixture over medium heat for 5-7 minutes, stirring occasionally, until the blueberries start to break down and release their juices.

In a small bowl, create a slurry by mixing 1 tablespoon cornstarch with 2 tablespoons water. Stir this mixture into the blueberry sauce, and continue cooking for another 3-4 minutes until the jam thickens. Remove the pan from the heat and let the jam cool to room temperature.

Step 3: Form and Bake the Cookies

After chilling the dough for about 15-20 minutes in the refrigerator, scoop tablespoon-sized portions of dough and roll them into balls. Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.

Using your thumb, gently press an indentation into the center of each dough ball to make space for the blueberry jam.

Spoon about 1 teaspoon of the cooled blueberry jam into the indentation of each cookie. Be careful not to overfill, as the jam can spill over during baking.

Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are lightly golden and the cookies have set. Check the cookies after 10 minutes, and adjust the baking time if necessary.

Once baked, remove the cookies from the oven and let them cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.