Combine marshmallows and boiling water in top of a double boiler. Cook over boiling water, stirring frequently, until marshmallows melt. Stir in banana and lemon juice. Cook an additional 2 minutes or until mixture is smooth and creamy. Remove from heat, and chill until slightly thickened.
Fold whipped cream into chilled mixture; spoon into two 2-cup metal ice trays (with dividers removed). Freeze until firm. Let stand 5 minutes before serving. Slice into bars or scoop out with an ice cream scoop, and serve in individual dessert dishes.