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, using your fingers or a fork, rub together until the mixture is crumbly. Set aside.
03 - In a medium bowl, whisk flour, baking powder, baking soda, and salt.
04 - In a large bowl, cream together the softened butter and sugar until light and fluffy, about 2–3 minutes.
05 - Beat in the egg and vanilla extract until combined.
06 - Add the dry ingredients to the butter mixture in two additions, alternating with milk, mixing until just incorporated. Do not overmix.
07 - Gently fold in blueberries.
08 - Scoop dough (about 2 tablespoons per cookie) onto prepared sheets, spacing 2 inches apart.
09 - Sprinkle each cookie mound generously with streusel topping.
10 - Bake for 13–15 minutes, or until edges are set and tops are lightly golden.
11 - Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.