a 9- or 10-inch cast iron skillet; a stand mixer or electric hand mixer
Preheat your oven to 350 degrees F.
For the topping: Melt together the brown sugar, butter, cinnamon, salt and allspice over medium-high heat in a 9- or 10-inch cast-iron skillet, stirring until the sugar has dissolved. Bring to a bubble and let the syrup cook until thick, about 5 minutes. Remove the pan from the heat and carefully stir in the dark rum (see Cook's Note) and vanilla, then return the pan to the heat for 1 minute. Arrange the banana slices cut side down on top of the syrup.
For the cake: Whisk together the flour, baking powder, salt, baking soda and nutmeg in a large bowl.
Beat together the brown sugar, granulated sugar and butter in a stand mixer or electric hand mixer until light and fluffy, about 2 minutes. Add the eggs, one at a time, beating well and scraping the bowl down after each addition. Beat in the mashed bananas and vanilla until incorporated. Beat in the dry ingredients, alternating with the buttermilk in two additions, scraping down the bowl to ensure all the dry ingredients are incorporated.
Carefully, spread the batter over the top of the bananas in the skillet. Place a baking sheet on the bottom rack of the oven to catch any spillage! Bake until a tester comes out with some moist crumbs sticking to it, 45 to 50 minutes. Let cool for 5 minutes before running a paring knife along the sides and inverting onto a serving platter while still warm.
If you would like to flambe the rum, do so off the heat. It is not necessary for the baking process.
Use extreme caution when igniting alcohol. Remove the pan from the heat source before adding the alcohol. Pour the alcohol into the pan and carefully ignite with a match or click lighter. Return the pan to the heat and gently swirl to reduce the flames.