In medium saucepan, combine chocolate and half and half. Cook over low heat until mixture is smooth and blended, stirring frequently. Add sugar and cook 2-3 minutes longer, until sugar is dissolved. Add peanut butter and vanilla and stir until blended. Remove from heat and pour into fondue pot or chafing dish. Serve with slices of fruit, angel food or pound cake, and marshmallows for dipping.