01 - Combine all-purpose flour, brown sugar, cold cubed butter, cinnamon, and salt in a medium bowl. Using a fork or pastry cutter, blend the butter into the dry ingredients until pea-sized crumbs form. Refrigerate until ready to use.
02 - In a large mixing bowl, beat softened butter with brown sugar and granulated sugar until light and fluffy, about 2–3 minutes.
03 - Add the egg and vanilla extract to the butter mixture. Beat until well combined.
04 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt.
05 - Gradually add the flour mixture to the wet ingredients, mixing just until combined. Take care not to overmix.
06 - Gently fold in the fresh blueberries, ensuring minimal breakage.
07 - Portion rounds of dough and arrange spaced apart on a parchment-lined baking sheet.
08 - Top each dough round with a spoonful of blueberry jam and use a knife or toothpick to gently swirl it into the surface.
09 - Remove streusel from the refrigerator and generously sprinkle over each dough portion, gently pressing crumbs onto the tops.
10 - Bake in a preheated oven at 375°F for 12–15 minutes, or until lightly golden but centers remain soft.
11 - Allow cookies to cool completely before serving. Enjoy slightly warm if desired.