Yes, you can use bananas as a natural sweetener to replace sugar in the Chocolate Peanut Butter Muffins recipe. Bananas will not only add sweetness but also moisture, which can make your muffins extra soft and tender. To replace the sugar in this recipe, you would need approximately 1 cup of mashed ripe bananas (about 2-3 medium bananas). However, please note that using bananas may change the overall taste and texture of your muffins, making them more banana-flavored and a bit denser.