Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a large bowl, cream the softened butter and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition.
Mix in the mashed bananas, pumpkin puree, and vanilla extract. In a separate bowl, whisk together the flour, baking soda, baking powder, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet mixture, alternating with sour cream. Stir until just combined. Be careful not to overmix.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Notes
If you prefer, you can add chopped walnuts or chocolate chips to the batter for extra texture and flavor.