In a small bowl, combine 3 tablespoons of softened butter, 2 tablespoons of brown sugar, 2 tablespoons of granulated sugar, ⅓ cup of flour, a pinch of salt, and a pinch of cinnamon. Use a fork or your fingers to mix until the mixture resembles coarse crumbs. Set this delightful streusel aside while you prepare the cookie dough.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, cream together ½ cup of softened butter, ⅓ cup granulated sugar, and ¼ cup brown sugar until light and fluffy. Add in 1 large egg and 1 teaspoon of vanilla extract, mixing until smooth and well combined.

Gently add in 1½ cups of all-purpose flour, 1 tablespoon of corn starch, ¼ teaspoon baking powder, ½ teaspoon baking soda, and ¼ teaspoon salt to the wet mixture. Mix until just combined.

Carefully fold in ½ cup of fresh blueberries, ensuring they are evenly distributed without crushing them. Drop 1 to 1.5 tablespoons of blueberry jam into the batter, and gently swirl it with a spatula.

Using a cookie scoop or spoon, drop equal portions of the dough (about 8) onto the prepared baking sheet and sprinkle the reserved streusel topping over each cookie mound.

Place the baking sheet in your preheated oven and bake the cookies for 13-14 minutes, or until the edges turn golden and the centers look set but still soft.

Once baked, remove the tray from the oven and let the cookies cool on the baking sheet for 15-20 minutes. Transfer the cookies to a wire rack to cool completely before indulging.