Place sweet potatoes in a heavy 5- to 6-quart saucepan; add enough water to cover. Bring to a boil over high heat. Reduce heat; simmer until potatoes are very tender, 20 minutes.
Immediately drain well; cool slightly.
Mash cooled potatoes.
In a medium bowl, combined mashed sweet potatoes, 1/4 cup butter or margarine, salt, brown sugar, nutmeg, cinnamon and egg yolks. If mixture is too soft to shape, refrigerate until chilled.
Flatten 1/3 cup sweet potato mixture into a 3-inch circle; place a marshmallow in center. Gather sweet potato mixture around marshmallow to enclose, leaving a small hole at top. Set aside.
Repeat with remaining sweet potato mixture and marshmallows.
In a medium bowl, combine corn flake crumbs with remaining 1/4 cup butter or margarine.
Roll puffs in buttered crumbs, turning to coat well.
Place on a baking sheet; freeze until solid.
If not baking immediately, place frozen puffs in a plastic freezer container; store up to 2 months.