01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Combine flour, brown sugar, and cinnamon in a small bowl. Add cold cubed butter and rub together with fingers or fork until crumbly. Set aside.
03 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
04 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
05 - Beat in egg and vanilla extract until fully combined.
06 - Add dry ingredients to butter mixture in two additions, alternating with milk. Mix until just incorporated, being careful not to overmix.
07 - Gently fold blueberries into the dough until evenly distributed.
08 - Scoop approximately 2 tablespoons of dough per cookie onto prepared baking sheets, spacing 2 inches apart.
09 - Sprinkle each cookie mound generously with 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.