Preheat the oven to 180C/350F. Grease a 12-count muffin tin and fill them with muffin liners. Set them aside.
In a large mixing bowl, whisk together the bananas, butter, maple syrup, eggs, and vanilla extract, until smooth. Add the dry ingredients and mix until combined. Fold through the walnuts and chocolate chips, if using them.
Distribute the batter evenly amongst the muffin tin and bake for 17-19 minutes, or until a skewer comes out mostly clean.
Remove the muffins from the oven and let them cool in the pan for 10 minutes, before transferring to a wire rack to cool completely.
* Plain all-purpose flour can also be used. If you use the gluten free flour, be sure it has added xanthan gum in it. TO STORE: Muffins can be stored in an airtight container at room temperature for up to five days. They will keep longer if you store them in the refrigerator.TO FREEZE: Place leftovers in a ziplock bag and store them in the freezer for up to 6 months.