01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a small bowl, mix flour, brown sugar, and cinnamon. Add cold butter and rub together with fingers or fork until crumbly. Set aside.
03 - In a medium bowl, whisk flour, baking powder, baking soda, and salt until well blended.
04 - In a large bowl, beat softened butter and granulated sugar until light and fluffy, about 2–3 minutes.
05 - Beat in the egg and vanilla extract until fully incorporated.
06 - Add dry ingredients to butter mixture in two additions, alternating with milk, mixing until just combined. Avoid overmixing.
07 - Gently fold blueberries into the batter until evenly distributed.
08 - Drop approximately 2 tablespoons of dough per cookie onto prepared sheets, leaving 2 inches of space between each.
09 - Generously sprinkle each cookie mound with the prepared streusel topping.
10 - Bake for 13–15 minutes until edges are set and tops are lightly golden.
11 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.