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.
Cream Cheese Topping
In a separate bowl, cream together cream cheese, granulated sugar and vanilla, until smooth.
Portion about 1Tbsp of cream cheese mixture onto the top of each unbaked muffin (~1Tbsp per muffin). Use a butter knife to gently cut in/swirl the cream cheese mixture. The mixture should not be swirled all the way through the muffin, but just through the muffin-top.
In another medium sized bowl, combine flour and brown sugar. Cut in butter using pastry knife.
Evenly sprinkle streusel over each muffin.
Bake on 350 degrees F for about 25 minutes, or until toothpick inserted in center (and not through blueberry) comes out mostly clean.