In a large, heavy saucepan melt marshmallows and butter over low to medium heat. Stir occasionally until fully melted.

Meanwhile, place popped popcorn in a very large mixing bowl. Carefully check the popcorn and remove any unpopped kernels.

Pour the melted marshmallows over the popcorn and mix well until popcorn is evenly coated with marshmallows.

Transfer mixture to a 9 x 13 inch pan. Press into pan. Using a damp spoon to smooth the top helps to be less sticky. Refrigerate until set.

Cut into 12 large or 24 smaller squares.