Preheat oven to 375°F. Grease a 12-cup standard muffin pan liberally with cooking spray.

To make the topping, add 1/2 tablespoon butter and 2 teaspoons brown sugar to each prepared muffin cup. Bake until melted and bubbly, about 10 minutes. Remove from oven and let cool slightly. Peel the bananas and slice into thin rounds. Place about 2 slices on the bottom of each muffin cup and about 2 to 3 slices along the sides of each.

To make the batter, in a large bowl, whisk together the flour, baking powder, cinnamon, nutmeg, salt, and dark brown sugar. In a glass measuring cup or separate bowl, whisk together the oil, eggs, milk, rum, and vanilla until well combined. Make a well in the center of the dry ingredients and pour in the wet ingredients. Stir with a wooden spoon until just combined, making sure not to over-mix. Fold in the chocolate chips until evenly dispersed.

Divide the batter evenly among each muffin cup, filling them up as much as possible. Give the pan a couple of taps on the counter to dislodge any air bubbles. Bake until puffed, and fully cooked through, about 18 minutes. A toothpick should come out clean when inserted in the center. Remove from the oven and run a knife along the sides of each muffin. Place a baking sheet over the muffins and invert immediately. Let cool slightly before serving warm. Enjoy!