Preheat the oven to 425°F, 220°C, or gas mark 7 with the cast iron skillet inside.
In a food processor or blender add the eggs, milk, flour, vanilla, salt, and sugar and blend until batter is evenly smooth.
Once the oven is hot remove the cast iron skillet and add the butter. Make sure all of the butter has melted. Immediately pour the batter into the skillet and place back into the oven to bake for 15 minutes or until the sides have started turning dark brown.
Making the bananas foster
While the Dutch baby is cooking in a sauce pan over medium high heat, heat the butter, brown sugar, and cinnamon.
Once the butter has melted add the bananas and cook for 1-2 minutes until they start to soften.
Add the vanilla and rum and with a long match or lighter, ignite the liquid. The flames will be high so stand back until they die down.
Remove from the heat and set aside.
Assembly
Remove the Dutch baby from the oven, pour the bananas foster on top and serve immediately. Serve with ice cream or whipped cream.