Preheat oven to 375 degrees F and line a standard 12-cup muffin tin with paper liners or spray with nonstick cooking spray.
Cream the butter and sugar until pale and light. Add the eggs and beat well and then beat in the milk, lemon juice and vanilla.
Stir in the flour, salt, and baking powder until just moistened. Fold in the blueberries and mashed bananas.
Divide the batter evenly among the muffin cups (I like to use a cookie or ice cream scoop). Sprinkle the tops with additional sugar if desired for a sweet crust.
Bake for 30 to 35 minutes, or until the muffins are baked through. Let cool.
Notes
Storing the Muffins:
Store the muffins at room temperature for 1 to 2 days. They also freeze well.