Preheat your oven to 400°F. Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the flour, brown sugar, baking soda, salt, and cinnamon.
In a medium bowl, combine the milk, melted butter, egg, and vanilla extract. Whisk until smooth.
Pour the wet ingredients into the dry ingredients. Stir gently until just combined. Overmixing can lead to tough muffins.
Gently fold in the blueberries.
Fill each cup of the muffin tin about 2/3 full of the batter.
In a small bowl, combine the brown sugar, cinnamon, softened butter, and flour. Use a fork to mix until crumbly.
Sprinkle the cinnamon streusel topping over top of each muffin.
Place the muffin tin in the preheated oven and bake for 20-25 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the pan for about 5 minutes before transferring to a wire rack.