Blueberry Muffin Cookies with Crumb Topping

Preheat oven to 350°F. Line two baking sheets with parchment paper or silicone baking mats.
Make the crumb: In a small bowl, stir together flour, granulated sugar, brown sugar, cinnamon, and salt. Drizzle in melted butter and toss with a fork until clumps form. Refrigerate while you prepare the dough.
Whisk melted, cooled butter with granulated sugar and brown sugar in a large bowl until combined and slightly lightened. Whisk in the egg, vanilla, and sour cream until smooth.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Add the dry ingredients to the wet and stir with a spatula just until no dry streaks remain.
Gently fold in the blueberries, taking care not to crush them (if using frozen, work quickly to avoid streaking).
Scoop dough into 2-tablespoon mounds and place 2 inches apart on the prepared sheets. Top each mound generously with crumb mixture, pressing lightly so it adheres.
Bake one sheet at a time for 11–13 minutes, until edges are set and centers look just baked. Do not overbake for the soft, muffin-like texture.
Cool on the baking sheet for 10 minutes, then transfer cookies to a wire rack to cool completely.
For the glaze, whisk powdered sugar, milk, and vanilla until smooth and pourable. Drizzle over cooled cookies. Let the glaze set before serving.