01 - Preheat oven to 350°F (175°C). 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 combined.
04 - In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt until evenly distributed.
05 - Gently fold the dry ingredients into the wet mixture using a spatula. Stir until just combined—do not overmix, as this can 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. Rotate the pan halfway through for even baking.
09 - Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving or storing.