01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Steam or roast the cubed butternut squash until fork-tender, about 15 minutes if steaming or 25 minutes if roasting. Let cool completely, 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, sift together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt.
05 - Add the dry ingredients to the wet mixture and stir gently until just combined. Do not overmix to keep the muffins tender.
06 - Gently fold in chopped nuts, chocolate chips, or raisins if using.
07 - Divide the batter evenly among the 12 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 rest in the pan for 5 minutes, then transfer to a wire rack to cool completely.