Using mini-marshmallows might save you some cutting.
From a box sold in Columbiaville, Michigan.
Marshmallow Popcorn Balls
1 c. popcorn
1/4 c. butter
1 tsp. salt
1/2 lb. marshmallows
Pop corn – sprinkle w/ salt.
Melt butter in skillet.
Cut marshmallow in 1/4s.
Alternate layers of corn + marshmallows in skillet. Cover; heat slowly until marshmallows are partially melted. Mix well.
Form into balls.
Makes 9 – 2″.