01 - Preheat the oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners or lightly grease each cup.
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 thoroughly until well combined.
04 - In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt.
05 - Gradually fold the dry ingredients into the wet mixture using a spatula. Stir gently until just incorporated — avoid overmixing to keep the muffins tender.
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 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.