01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly with oil.
02 - In a large bowl, whisk together the butternut squash purée 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 purée mixture. Whisk until well combined and homogeneous.
04 - In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt until evenly distributed.
05 - Gently fold the dry ingredient mixture into the wet mixture, stirring until just combined. Take care not to overmix, as this will result in dense muffins.
06 - If using, gently fold in the chopped walnuts or pecans and dark chocolate chips until evenly distributed throughout the batter.
07 - Divide the batter evenly among the prepared muffin cups, filling each about two-thirds to three-quarters full.
08 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are lightly golden.
09 - Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.