Step 1: Make the Blueberry Jam

1. In a small saucepan, combine the blueberries, sugar, and lemon juice.
2. Cook over medium heat, stirring occasionally, until the blueberries break down and the mixture thickens (about 8–10 minutes).
3. Stir in the vanilla extract and cornstarch, and cook for another 1–2 minutes until the jam thickens further.
4. Remove from heat and let cool completely.

Step 2: Prepare the Cookie Dough

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3. In a large mixing bowl, beat the butter and sugar until light and fluffy (about 2–3 minutes).
4. Add the vanilla extract and egg, mixing until well combined.
5. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.

Step 3: Shape and Fill the Cookies

1. Scoop about 1 tablespoon of dough and roll it into a ball. Place it on the prepared baking sheet, spacing the cookies about 2 inches apart.
2. Using your thumb or the back of a small spoon, create an indentation in the center of each cookie.
3. Fill each indentation with about 1/2 teaspoon of blueberry jam.

Step 4: Bake

1. Bake the cookies for 12–15 minutes, or until the edges are lightly golden.
2. Remove from the oven and immediately sprinkle the cookies with a little granulated sugar for a sparkling finish.
3. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.