Preheat the oven to 350°F (175°C). Grease and flour a 10–12 cup bundt pan thoroughly.

1. In a large bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
2. In another bowl, mix mashed bananas, granulated sugar, brown sugar, eggs, melted butter, sour cream, and vanilla extract until smooth.
3. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Do not overmix.
4. Pour half the batter into the prepared bundt pan and smooth the top.
5. Add your swirl layer by sprinkling cinnamon sugar or drizzling melted chocolate/Nutella over the batter.
6. Pour the remaining batter on top and gently swirl with a knife or skewer for a marbled effect.
7. Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
8. Let the cake cool in the pan for 15 minutes, then invert onto a wire rack to cool completely.