Preheat oven to 350º. Lightly grease a muffin tin with cooking spray or vegetable oil, or line with paper muffin liners.
Prepare streusel topping and set aside.
In a small bowl, whisk together flour, baking powder and salt and set aside.
In a large bowl, beat butter and sugar until light and fluffy. Add eggs, vanilla and milk. Add flour mixture and stir until just combined. Stir in blueberries.
Divide batter evenly into 12 muffin cups. Sprinkle muffins with streusel topping.
Bake at 350º for 20 to 25 min, until a tester inserted into the center comes out clean. When muffins are done, cool for a few minutes in the muffin pan before removing to cool on a wire rack.
Streusel ToppingIn a small bowl, whisk together sugar and flour. Add oil and mix until mixture has a sandy texture.