01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Steam or roast the butternut squash until fork-tender, about 15 minutes if steaming or 25 minutes if roasting. Let cool, then mash to yield 1 cup of purée.
03 - In a large bowl, whisk together the eggs, oil, maple syrup, mashed bananas, butternut squash purée, and vanilla extract until smooth and well blended.
04 - In a separate bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt until evenly distributed.
05 - Add the dry ingredient mixture to the wet ingredients. Stir gently with a spatula until just combined; avoid overmixing to keep muffins tender.
06 - Gently fold in chopped nuts, chocolate chips, or raisins if desired.
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.