Preheat the Oven: Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it with nonstick spray.
Mix Wet Ingredients:
In a large bowl, whisk together the mashed bananas, pumpkin puree, granulated sugar, brown sugar, oil, eggs, and vanilla extract until smooth.
Combine Dry Ingredients:
In another bowl, whisk together the flour, baking soda, cinnamon, nutmeg, and salt.
Mix Wet and Dry Ingredients:
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.
Fold in Optional Mix-Ins:
If using nuts or chocolate chips, gently fold them into the batter.
Fill the Muffin Tin:
Spoon the batter evenly into the prepared muffin tin, filling each cup about 3/4 full.
Bake:
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool and Serve:
Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.