In a mixing bowl, combine flour, brown sugar, cold cubed butter, cinnamon, and a pinch of salt. Use a fork or your fingers to blend the mixture until it resembles coarse crumbs, about 5 minutes. Once crumbly, cover the bowl and refrigerate.

In a large mixing bowl, beat the softened butter and both sugars together until light and fluffy, approximately 3-5 minutes. Add in one egg and the vanilla extract, mixing until fully combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt until well blended. Gradually incorporate this dry mix into the wet ingredients, stirring gently until just combined.

Carefully fold the fresh blueberries into the cookie dough, taking care not to break them. Gently turning the mixture with a spatula will help maintain the berries’ shape while evenly distributing them throughout.

Line a baking sheet with parchment paper to prevent sticking. Using a cookie scoop or two spoons, portion out the dough onto the sheet, leaving adequate space between each mound. Add small dollops of blueberry jam onto each dough round, swirling slightly.

Retrieve the chilled streusel from the fridge and sprinkle a generous amount atop each cookie dough portion, ensuring it's evenly distributed.

Preheat your oven to 375°F (190°C). Bake the cookies for 12-15 minutes or until the edges are lightly golden while the centers remain soft.

Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for about 5 minutes. Transfer them to a wire rack to cool further.