01 - Preheat oven to 350°F. Line a 12-cup muffin pan with paper liners or lightly grease each cup with baking spray.
02 - In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly distributed.
03 - In a medium bowl, beat the eggs with the brown sugar and granulated sugar until smooth and well combined. Stir in the oil, vanilla extract, mashed bananas, and butternut squash puree.
04 - Pour the wet mixture into the dry ingredients and fold gently until just combined. Do not overmix to keep muffins tender.
05 - If desired, fold in the chopped nuts or chocolate chips until evenly distributed throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
07 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
08 - Let the muffins rest in the pan for 5 minutes, then transfer them to a wire rack to cool completely before serving.