01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners or grease each cup lightly with oil.
02 - In a large mixing bowl, whisk together the butternut squash puree 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 and homogeneous.
04 - In a separate medium bowl, sift together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt.
05 - Gradually add the dry ingredient mixture to the wet ingredients. Fold gently with a spatula until just combined — do not overmix, as this will produce tough muffins.
06 - If using, gently fold in the chopped nuts and/or dark chocolate chips until evenly distributed throughout the batter.
07 - Divide the batter evenly among the 12 prepared muffin cups, filling each about three-quarters full.
08 - Bake for 22–25 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are lightly golden.
09 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving or storing.