01 - Preheat the 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 purée and mashed banana until the mixture is smooth and free of lumps.
03 - Add the eggs, melted coconut oil, maple syrup (or honey), and vanilla extract to the purée base. Whisk until all wet ingredients are thoroughly combined.
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 - Pour the dry ingredient mixture into the wet ingredients. Fold gently with a spatula until just combined; avoid overmixing to keep the muffins tender.
06 - If using, fold the chopped nuts or dark chocolate chips into the batter with a few gentle strokes.
07 - Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds 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 them to a wire rack to cool completely before serving.