Preheat oven to 425°F (not convection/fan assisted) with rack in centre of oven. Take your milk and eggs from the fridge to warm to room temperature.
Make the Batter: Melt the 2 Tbsp butter in the microwave or a small saucepan. Pour into a blender or medium bowl and let cool slightly, if hot. Add the milk, eggs, flour, white sugar, vanilla extract, and salt. Blend batter until smooth. Let sit in blender for 30 minutes, while oven preheats and you prepare the other ingredients. (Make Ahead: you can refrigerate the batterin an air-tight container up to 24 hours. Bring to room temperature before using).
Peel bananas and slice in half length-wise. Set aside.
Heat a 10-inch top-diameter cast-iron skillet (or similar sized oven-proof skillet) over medium-high heat on the stovetop for 5-10 minutes. Add the remaining 2 Tbsp of butter and stir to melt. Once butter is foaming, add the brown sugar and stir to combine. Add the halved bananas, round side down. Swirl in pan, then flip the bananas so the flat side of the bananas is down. Quickly re-blend or re-whisk your resting batter, then pour the batter all at once into the hot skillet. Immediately transfer the skillet to the preheated oven and bake for 18-22 minutes, or until puffy and golden. *Don't open the oven door while the Dutch baby is baking. You can monitor it's progress with the oven light on and through the glass.
Remove from oven. Pancake will deflate as it cools. Dust with icing sugar, if desired. Top with ice cream and serve warm from the skillet. *Remember the cast-iron handle is hot. Wrap with a tea towel to avoid someone accidentally grabbing it.
*Nutritional information does not include ice cream topping.The brown sugar/buttersauce will be absorbed into the Dutch baby as it bakes, so you won't have a sauce at the bottom of the skillet (all the flavour will be in the pancake though!).Dutch babies puff up as they cook, but will deflate soon after removing from the oven. This is normal and to be expected.If you feel like boozing this one up, remove your skillet from the heat (before transferring to the oven) and add a splash of rum or brandy before pouring the batter in.Be sure to read the “Ingredient and Cook's Notes" (above the recipe card!), where I share more detailed tips, variations and substitution suggestions for this recipe!