For best results, use very ripe bananas – the browner, the better! Feel free to substitute other nuts or dried fruits for the walnuts and chocolate chips. Store leftover muffins in an airtight container at room temperature for up to 3 days, or freeze for longer storage. For a richer flavor, try using melted butter instead of coconut oil. If you don't have steel-cut oats, you can use rolled oats pulsed in a food processor until finely ground.