Preheat oven to 350 F
To make the cookies, whisk the flour, baking powder, and salt together in a medium bowl and set aside.
With a handheld or stand mixer, beat the butter, granulated sugar, and brown sugar together until creamy.
Add the egg and vanilla extract and beat on low speed until combined.
Alternate adding the dry ingredients and milk, and beat everything until it's just combined.
With a wooden spoon or rubber spatula, gently fold the blueberries in by hand.
To make the crumble topping, combine all ingredients in a small bowl and rub together using your hands until course crumbs form.
Scoop cookie dough by the tablespoon and place 2-3 inches apart on a baking sheet lined with parchment paper.
Gently flatten each cookie dough ball and top with a generous sprinkling of crumble topping.
Bake for about 15 minutes or until edges turn golden brown and the top of the cookie springs back when touched.