Use a potato masher to mash bananas until smooth. Place mashed bananas in a 5-quart heavy pot. Stir in the remaining ingredients. Bring to boiling; reduce heat. Simmer, uncovered, about 20 minutes or until thickened, stirring frequently.
Place pot of banana butter in sink filled with ice water; stir to cool. Ladle banana butter into clean, wide-top half-pint freezer containers, leaving a 1/2-inch headspace. Seal and label. Freeze up to 3 months. Thaw in the refrigerator before serving.