Lightly butter the bottom and sides of a 13-by-9-inch square baking pan (halve this recipe for an 8-by-8-inch pan). Line bottom and long sides with a strip of aluminum foil; lightly butter foil.In a large saucepan, melt butter and molasses over medium-high heat. Stir in salt and spices. Add 6 cups of mini marshmallows and stir until melted and smooth. Remove from heat, then fold in rice krispies until evenly coated (be somewhat gentle with them so they don’t crush). Fold in remaining 2 cups marshmallows.Spread into prepared pan, pressing into an even layer. If it helps, lightly butter your hands or the spatula and use that to press the mixture into the pan without sticking. If you want to top the bars with sprinkles, do so while they are still slightly warm and sticky.Let cool completely before slicing into bars.