Preheat oven to 350 degrees F
In medium bowl, combine melted, cooled butter, sugar, and vanilla. Stir well.
Add in egg whites and egg, being sure to beat the eggs in well.
Add milk and stir.
In a separate bowl, combine flour, baking powder and salt.
Gently fold the flour mixture into the wet ingredients (it will take approximately 12 folds of your spatula to properly mix the batter without over-mixing. Once you have done the 9th fold, add in your blueberries and continue to fold in.
Pour batter into muffin cups, filling 3/4 full.