01 - Preheat the oven to 350°F. Line a standard 12-cup muffin tin with paper liners or grease each cup lightly with oil.
02 - In a large mixing bowl, whisk together the butternut squash purée and mashed banana until the mixture is smooth and uniform.
03 - Add the eggs, melted coconut oil, maple syrup (or honey), and vanilla extract to the purée mixture. Whisk thoroughly until well combined.
04 - In a separate medium bowl, sift together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt.
05 - Gradually add the dry ingredient mixture to the wet ingredients. Fold gently with a spatula until just combined — avoid overmixing to keep the muffins tender.
06 - If using, gently fold the chopped nuts and/or dark chocolate chips into the batter.
07 - Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
08 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are lightly golden.
09 - Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.