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 your fingers or a fork until the 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, about 2 to 3 minutes.
05 - Beat in the egg and vanilla extract until fully combined.
06 - Add dry ingredients to butter mixture in two additions, alternating with milk. Mix until just incorporated. Do not overmix.
07 - Gently fold blueberries into the batter. If using frozen blueberries, add them directly without thawing.
08 - Scoop about 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 to 15 minutes, or until edges are set and tops are lightly golden.
11 - Let cookies cool on baking sheet for 5 minutes, then transfer to wire rack to cool completely.