01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners and set aside.
02 - Steam or roast the cubed butternut squash until fork-tender, approximately 15 minutes if steaming or 25 minutes if roasting. Allow to cool, then mash thoroughly to yield 1 cup of smooth purée.
03 - In a large mixing bowl, whisk together the eggs, vegetable oil, maple syrup, mashed bananas, butternut squash purée, and vanilla extract until the mixture is smooth and well blended.
04 - In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, ground cinnamon, nutmeg, and salt until evenly distributed.
05 - Gradually add the dry ingredient mixture to the wet ingredients. Stir gently with a spatula until just combined, being careful not to overmix.
06 - Gently fold in the chopped walnuts or pecans and chocolate chips or raisins, if using, until evenly distributed throughout the batter.
07 - Divide the batter evenly among the 12 prepared muffin cups, filling each approximately three-quarters full.
08 - Bake on the center rack for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
09 - Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.