Preheat a 12-inch cast iron skillet (or other similarly sized oven-safe pan or dish) in a 425°F oven.
While oven and skillet preheats, blend eggs, milk, flour, sugar, vanilla extract, and sea salt in a blender until just combined. Optionally, whisk by hand. Work the batter until lumps are dissolved, taking care not overwork. (You can work too much air into the batter if you blend or whisk too much.)
Once the oven and skillet are preheated, remove the skillet from the oven and add the unsalted butter to melt.
Pour batter into the hot skillet and bake for 20-minutes until golden brown and puffed up.
Banana Caramel Topping
While the Dutch Baby is baking, melt butter in a large saucepan or skillet over medium-high heat. Add water and sugars, stirring until melted and bring to a boil. Reduce heat to medium and simmer for 8 to 10-minutes until thickened. Dragging a spoon or spatula across the pan should leave a trail.
Add banana slices, nutmeg, cinnamon, a pinch of kosher salt and cook for 2 to 3-minutes until bananas have softened, but are not falling apart. Keep warm over low heat until Dutch baby is done cooking.
Bananas Foster Dutch Baby Assembly
Remove the Dutch Baby from oven and rest 2 to 3-minutes. Add several pats of butter and allow melt over the top.
Top with Banana Caramel Topping, drizzle with maple syrup, and sprinkle with powdered sugar for garnish.