Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
Add the milk and vanilla extract and stir until just combined.
Gently fold in the blueberries.
Fill each muffin liner about 2/3 full with batter.
In a separate bowl, combine the flour, brown sugar, and cold butter. Use a pastry blender or your fingers to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the muffin batter.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Notes
For a crispier topping, broil the muffins for the last minute of baking. Store leftover muffins in an airtight container at room temperature.