Preheat the oven: Preheat your oven to 350°F (175°C).
Prepare the batter: In a bowl, mix the mashed bananas, eggs, melted butter, dark rum, and vanilla extract.
Combine dry ingredients: In another bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon.
Mix wet and dry ingredients: Combine the wet ingredients with the dry ingredients until just mixed.
Prepare the streusel: In a separate bowl, mix brown sugar, oats, chopped hazelnuts, and a bit of melted butter.
Layer the cake: In a greased baking pan, pour half of the batter, sprinkle half of the streusel, then add the remaining batter and top with the rest of the streusel.
Bake: Bake in the preheated oven for about 45-50 minutes or until a toothpick comes out clean.
Cool and serve: Allow to cool before slicing and serving.