Preheat oven to 350 degrees F. In a microwave safe bowl, combine the butter and hot pepper sauce. Melt in the microwave in 30 second increments until the butter is melted. Mix in the garlic salt.
In a large bowl, combine the popcorn and the buffalo sauce until well coated. Pour into two 13x9 pans or large baking dishes. Bake for 15 to 18 minutes.