Preheat oven to 325° F. Lightly grease a jumbo muffin tin (6 count) and set aside. (I use Bak-Klene to prepare my pans - you can get it on Amazon or Williams Sonoma).
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 it begins to simmer. Turn the heat down to very low and let simmer for 3-5 minutes. Remove from heat and let slightly cool. It does not have to cool all the way down, but just so it is not super hot.
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, ground flaxseed, baking soda, salt, nutmeg, cinnamon, and cardamom.
Add the dry ingredients to the wet banana ingredients and gently fold until combined. Pour this mixture into the prepared muffin tin filling about 3/4 of the way high. Bake for ~25 minutes, or until a toothpick comes out clean from middle of the muffins! Makes 6-7 jumbo large muffins.
While the muffins cool, you can prepare the frosting. Whisk powdered sugar, rum and butter together. Make sure your muffins are 100% cooled down before you frost them!! Enjoy!!!