Preheat your oven to 375°F. Lightly coat your mini muffin pan with nonstick spray and set it aside.

In a large bowl, use a hand mixer to combine sugar, butter, egg, vanilla, and milk. Once well mixed, add the flour, baking powder, and salt, and continue mixing. Gently fold in the blueberries with a spoon to avoid mashing them.

Spoon the batter evenly into the muffin pan, which will yield about 16 mini muffins.

To make the streusel topping, mix flour, brown sugar, and cinnamon in a small bowl. Cut in the butter using a fork until the mixture becomes crumbly. Sprinkle the streusel evenly over the muffins.

Bake for 12-13 minutes, or until the muffins' edges are lightly golden brown. Remove from the oven and let cool before serving.