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 mixture is crumbly. Set aside.
03 - In a medium bowl, whisk together 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, approximately 2-3 minutes.
05 - Beat in the egg and vanilla extract until fully incorporated into the butter mixture.
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 batter until evenly distributed. If using frozen berries, add directly from freezer without thawing.
08 - Scoop approximately 2 tablespoons of dough per cookie onto prepared baking sheets, spacing cookies 2 inches apart.
09 - Sprinkle each cookie mound generously with prepared streusel topping, pressing lightly to adhere.
10 - Bake for 13-15 minutes until edges are set and tops are lightly golden brown.
11 - Let cookies cool on baking sheet for 5 minutes before transferring to wire rack to cool completely.