1Heat oven to 300^. Line a large shallow roasting pan with heavy foil, extending foil over edges of pan. Butter or spray foil.
Remove all unpopped popcorn. Pour popcorn into prepared pan. Keep warm in oven.
Combine sweetened condensed milk and dry gelatin in medium saucepan. Heat and stir over low heat until mixture is slightly thickened and bubbly, about 5 minutes.
Pour mixture over popcorn. Stir gently to coat using a long handled wooden spoon. Bake 20 minutes, stirring every 5 minutes. Carefully turn out onto large pieces of lightly buttered foil. Cool. Break into pieces or clusters.