01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease each cup lightly with oil.
02 - In a large bowl, whisk together the butternut squash purée and mashed banana until smooth and free of lumps.
03 - Add the eggs, melted coconut oil, maple syrup (or honey), and vanilla extract to the squash-banana mixture. Whisk until fully combined.
04 - In a separate medium bowl, sift together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt until evenly distributed.
05 - Pour the dry ingredient mixture into the wet ingredients. Stir gently with a spatula until just combined — avoid overmixing to keep muffins tender.
06 - Fold in the chopped nuts and/or dark chocolate chips, if using, distributing them evenly throughout the batter.
07 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
08 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
09 - Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.