For the Caramelized Banana Topping:

1. Preheat the oven to 350°F (180°C). Grease a 9×13 inch (23x33cm) baking pan with 1 tbsp of melted butter. Use a pastry brush to thoroughly coat the sides and bottom of the pan.
2. In a small pan over medium heat, add the 70 g (5 tbsp) of butter, dark brown sugar, and rum. Stir constantly to melt everything together.
3. Once melted, stir frequently and cook until the mixture is thick, glossy, and slightly bubbly.
4. Immediately remove it from the heat and pour the sauce evenly into the prepared baking pan. Use a silicone spatula to spread it toward the edges as best as you can.
5. Cut the bananas in half lengthwise and place the halves on the sauce, cut side down. Set aside.

For the Cake:

1. In a medium bowl, whisk together the flour, cinnamon, baking powder, and salt. Set aside.
2. In a large bowl, whisk together the melted and cooled butter with the dark brown sugar. Add the mashed bananas, eggs, sour cream, rum, and vanilla extract. Whisk until fully combined.
3. Fold in half of the flour mixture using a silicone spatula until just combined. Add the remaining half of the flour along with the chopped walnuts and fold until just combined. Don’t overmix.
4. Pour the batter evenly into the prepared baking pan over the top of the bananas. Use a spatula to smooth out the top.
5. Bake for 30-40 minutes or until a toothpick inserted into the center comes out clean. The center of the cake should spring back when touched.
6. Cool in the pan for 10 minutes then place a cooling rack over the pan. Quickly and carefully flip it over and tap the top and sides to loosen any bits of cake that might be stuck (we suggest wearing oven mits since the pan will still be hot). Gently lift off the pan to reveal the cake.
7. Top with more walnuts if desired then slice and serve warm. Best enjoyed with a scoop of vanilla ice cream!