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Prepare the Streusel Topping:
In a small bowl, mix together the granulated sugar, flour, and cinnamon.
Cut in the cold butter using a fork or pastry cutter until the mixture becomes crumbly.
Set aside for later.
Prepare the Cookie Dough:
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the egg and vanilla extract, and beat until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Stir in the milk until the dough becomes smooth and slightly sticky.
Gently fold in the blueberries, being careful not to overmix.
Shape and Bake the Cookies:
Scoop out 1 1/2-inch balls of dough and place them onto the prepared baking sheet.
Sprinkle the streusel topping generously over each cookie.
Bake for 12-15 minutes, or until the cookies are lightly golden on the edges and set in the center.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Prepare the Glaze (Optional):
In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.
Drizzle the glaze over the cooled cookies.