- Preheat the oven to 350°F (175°C). Line a muffin tin with paper liners or grease it lightly with cooking spray. 
- In a large mixing bowl, combine the mashed bananas, grated zucchini, vegetable oil, brown sugar, granulated sugar, eggs, and vanilla extract. Mix until well combined. 
- In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon. 
- Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix. 
- If using, fold in the chopped nuts and chocolate chips. 
- Divide the batter evenly among the muffin cups, filling each about 2/3 full. 
- Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean. 
- Allow the muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.