Preheat the oven to 180 °C (350 °F) and line a muffin tin with paper cases or put some silicone moulds on a tray.
In a mixing bowl, mash the banana with a fork until smooth. Add the eggs and whisk until well combined.
Stir in the vanilla, oil, cinnamon, mixed spice and (if using) sugar.
In a separate bowl, mix together the flour, oat, and baking powder. Fold the dry ingredients into the banana mixture until just combined.
Gently stir in the grated carrot, chopped walnuts and sultanas.
Spoon the batter evenly into the muffin cases, filling them about ¾ full.
Bake for 20–25 minutes, or until a toothpick inserted into the centre comes out clean.
Transfer the muffins to a wire rack and let them cool before serving.