Make the dutch baby: Preheat oven to 375F. Place 2 tablespoons butter in a 9-in cast iron skillet and place skillet in oven to heat butter.
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. 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.
While dutch baby bakes, make the banana topping: In a medium skillet, saute bananas in butter, gradually adding sugar, cinnamon, cream, and salt. Stir occasionally 4-5 minutes until bananas are golden and sauce is thickened.
Fill baked dutch baby with banana topping and dust with powdered sugar. Slice into wedges and enjoy!