Preheat oven to 325° F.
Grease your metal 9x5 inch loaf pan: brush with softened butter and sprinkle an even layer of flour around the pan. Set aside.
In a medium skillet add the ripe mashed bananas, butter, brown sugar, and rum. Cook and whisk banana mixture over medium heat until butter melts and mixture begins to simmer. Turn the heat down to low and let simmer for ~3-5 minutes, whisking occasionally.
Remove from heat and let slightly cool for about 10 minutes. It does not have to cool all the way, but just so it is not super hot. (If you are folding in chocolate chips into the batter you will want it to cool completely).
In a large bowl, combine the eggs, oil, whole milk and vanilla extract. Add the cooled down banana mixture to this and whisk together.
In a separate mixing bowl, combine the flour, sugar, baking soda, salt, nutmeg, cinnamon, and cardamom. Add the dry ingredients to the wet ingredients and gently fold until combined. Do not over mix the batter.
Pour this mixture into the prepared pan. Bake for ~70-75 minutes, or until a toothpick comes out clean.
While the bread cools, you can prepare the frosting. Whisk powdered sugar, rum and butter together. Make sure your bread is 100% cooled down before you drizzle the frosting over the bread. Enjoy!