Put an unpeeled bananas on a foil-lined baking sheet. Bake them in a 300 F oven for 15 to 20 minutes, flipping halfway, or until the banana skins turn completely black all over. Allow to cool, remove banana from the peel and mash it for your recipe. Microwave:  This method will soften the banana but it doesn't sweeten it, you may need to add some extra sugar. This method works better with partially ripened fruit rather than fully green bananas. Peel the bananas and place them on a microwave-safe plate or bowl. Heat them on high just until they start to sizzle, about 30 seconds.