Melt butter in a large saucepan over low heat. Add marshmallows and stir until melted. Stir in vanilla and remove from heat.
Stir in cereal until well coated. Divide cereal among 1 greased (12-count) muffin. Press firmly into bottom and up sides creating a cup shape. Refrigerate 1 hour or until firm.
Dip rim of cup into melted chocolate. Let harden. Serve filled with ice cream.