Chocolate Marshmallow Peanut Butter Squares

Place first 5 ingredients in a large heavy saucepan; cook and stir over low heat until smooth and blended, about 5 minutes (mixture will be very thick). Remove from heat; stir in remaining ingredients. Spread into a greased 13x9-in. pan., Refrigerate, covered, until firm, about 4 hours. Cut into squares. Store in an airtight container in the refrigerator.