01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - Steam or roast the cubed butternut squash until fork-tender, approximately 15 minutes steaming or 25 minutes roasting. Allow to cool completely, then mash until smooth to yield 1 cup of 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, stirring gently with a spatula until just combined. Take care not to overmix to keep the muffins tender.
06 - If desired, gently fold chopped walnuts or pecans and chocolate chips or raisins into the batter until evenly distributed.
07 - Divide the batter evenly among the 12 prepared muffin cups, filling each about 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 - Let the muffins rest in the pan for 5 minutes, then transfer to a wire cooling rack to cool completely before serving.