• Preheat your oven to 425°F (220°C) conventional (see note 2 if using a convection oven with a fan). Line a 12-hole standard muffin tray with paper liners and set aside.

Walnut Crumble Topping

• Add the brown sugar, flour, chopped walnuts, and cinnamon to a small bowl and mix to combine.
• Add the melted butter and mix until the mixture becomes coarse and crumbly. You can use your hands towards the end to ensure everything is evenly mixed. Set aside.

Muffin Batter

• In a medium bowl, sift together flour, baking soda, baking powder, ground cinnamon, and salt. Whisk until well combined and set aside.
• In a large bowl, mash the ripe bananas until smooth.
• Add the brown sugar, egg, melted butter, oil, and vanilla, then whisk until fully combined.
• Add the dry ingredients to the wet mixture and gently fold together using a spatula until just combined. Be careful not to overmix.
• Fold in the chopped walnuts until evenly distributed. Do not overmix.
• Evenly divide the batter between the prepared muffin liners, then generously sprinkle the crumble topping over the top. There’s no need to press it in.
• Bake at 425°F (220°C) for 8 minutes, then without opening the oven, reduce the temperature to 350°F (180°C) and continue baking for 10–12 minutes, or until a toothpick inserted into the centre comes out clean or with a few moist crumbs.
• Allow the muffins to cool in the tray for about 15 minutes before transferring to a wire rack. Serve warm or let them cool completely before enjoying.