The crust of mochi muffins will soften over time due to the moisture. You can reheat them straight out of the refrigerator by baking them for 3 minutes at 350°F (175°C) in an oven, or for 1-2 minutes at 350°F (175°C) in an air fryer. This will revive the magical crust! If they've been frozen, add 3-5 minutes of reheating time.