In a large pot, melt butter over medium heat. Add the 10 ounce bag of marshmallows and salt. Cook and stir until just about completely melted/smooth. Remove from heat and add the cereal and remaining marshmallows. Mix until cereal is evenly coated. (If you want the additional marshmallows pretty intact, completely mix the cereal with the melted marshmallows before adding them in.)
Press into the butter dish and cool completely. Cut and serve.