01 - Preheat oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or lightly grease with baking spray.
02 - In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly distributed.
03 - In a medium bowl, beat eggs, brown sugar, and granulated sugar until well combined. Stir in oil, vanilla extract, mashed bananas, and butternut squash puree until smooth.
04 - Add wet ingredients to dry ingredients and fold gently until just combined. Do not overmix to keep muffins tender.
05 - Fold in chopped nuts or chocolate chips if using, distributing evenly throughout the batter.
06 - Divide batter evenly among the 12 muffin cups, filling each about ¾ full.
07 - Bake for 22–25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
08 - Cool muffins in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.