Preheat the oven to 325°. Grease and flour a bundt pan.
Make the filling: combine the filling ingredients and mix well with an electric mixer. Set aside. Rinse the beaters.
Make the cake batter: whisk together flour, baking soda, cinnamon, and salt. Set aside.
In a large mixing bowl cream together the butter and brown sugar using an electric mixer. Add the eggs one at a time. Add the banana, buttermilk, rum, and vanilla. Gradually beat in the dry ingredients.
Pour half the batter in the prepared pan. Using a teaspoon, add the filling by dollops halfway between the side of the pan and the center tube. Don't all the filling to touch the pan or the center tube. Spoon the rest of the batter evenly on top.
Bake for 70-75 minutes or until a toothpick inserted into the cake comes out clean. If adding a king cake baby, insert it now. Allow the cake to cool in the pan 5 minutes, then turn it out onto a baking rack to cool completely.
When the cake is cool, make the glaze: melt together butter and brown sugar. Off the heat stir in the rum, powdered sugar, and buttermilk. Pour over cake.
