• Preheat the oven to 350°F/180°C and line muffin pans with paper baking cups.
• Spray each liner bottom with a little bit of oil.

The streusel topping:

• Place sugar, flour, and butter in a bowl and blend with your hands until you get a crumbly mixture.

The muffins:

• In a mixer bowl with the paddle attachment, cream together butter, sugar, salt, and vanilla until light and fluffy.
• Add the eggs, one at a time, beating well after each addition.
• Sift together the flour and baking powder and add to the creamed mixture alternating with the milk.
• Once the mixture is smooth, stop mixing.
• Fold in blueberries.
• Fill muffin cups with about 2/3 cup muffin batter. Sprinkle the streusel evenly over the top of the muffins.
• Bake for 15-25 minutes until a toothpick comes out clean from the center of a muffin. Cool 5 minutes in the pan, then remove from pan and allow to cool completely on a rack.