In a medium bowl, whisk remaining 2 tablespoons melted butter, milk, eggs, sugar, and vanilla until smooth. Whisk in flour, and salt until a smooth batter forms. Gently fold in blueberries. Remove skillet from oven and carefully pour batter into the bottom. Place skillet back in oven and bake dutch baby at 375F 20-25 minutes or until puffed up and golden brown.