01 - Preheat oven to 350°F. Line a 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 incorporated.
04 - In a separate bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt until evenly distributed.
05 - Pour the dry ingredients into the wet mixture. Fold gently with a spatula until just combined — do not overmix or the muffins will become dense.
06 - If using, fold in the chopped nuts or dark chocolate chips until evenly distributed throughout the batter.
07 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
08 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
09 - Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely before serving.