Prick sweet potatoes several times with a fork, place in a baking pan, and bake at 350° for 45 to 60 minutes or until tender. Cool, peel, and place in a large bowl. Mash potatoes to a smooth puree. Stir in egg yolks and next 3 ingredients. Refrigerate 2 hours or until well chilled.
Divide sweet potato mixture into 24 portions. Form each into a ball with 1 mini marshmallow in the center. Roll each ball in cornflake crumbs. Place puffs in a lightly greased baking pan. Bake at 450° for 8 minutes or until lightly browned. Serve immediately.
Note: Puffs may be prepared ahead of time and frozen. Place puffs on a wax paper-lined pan, freeze overnight, then store in a resealable plastic bag. To prepare, place frozen puffs in a lightly greased baking pan coated with cooking spray, and bake at 450° for 12 minutes or until lightly browned.