Preheat oven to 250. Combine butter, brown sugar and light corn syrup in a 2-quart sauce pan. Cook on medium heat until mixture has melted. Add baking soda. (This will cause the mixture to foam). Pour puff corn in a large roaster pan. Pour caramel mixture on top and stir until mixed. Place in oven for 45 minutes. Stir at least every 10-15 minutes. Remove from oven. Pour on waxed paper and break apart. Cool.