Preheat your oven to 375° and set a rack to the middle position. Line the cups of a standard 12-cup muffin pan with paper liners or grease them lightly with butter.

First, make the streusel: In a small bowl, use a fork to combine the melted butter, flour, oats, brown sugar, and salt until crumbly. Set aside.

Now make the batter: In a large bowl, whisk together the flour, oats, baking powder, and salt. In a separate bowl, whisk together the melted butter, milk, sugars, egg, and vanilla.

Pour the wet ingredients into the dry ingredients, and stir, just until combined. Gently fold in the blueberries. Divide the batter evenly among the prepared muffin cups. Sprinkle the streusel topping evenly over each. Bake until the top is crisp and lightly golden, about 25 minutes. Let cool slightly before unmolding.