Line two 9x13-inch pans with parchment paper, leaving an overhang. Lightly spray with cooking spray and set aside.
In a large pot, melt 5 tablespoons butter over medium-low heat. Once melted, reduce heat to low and stir in 8 cups marshmallows. Stir constantly until just melted, then remove from heat. Remove the urge to increase the heat!
Remove from heat and stir in ¼ teaspoon salt and 6 cups Rice Krispies. Mix until evenly coated. Transfer to one prepared pan and gently press into an even layer (don’t press too firmly—keep them soft and chewy). Lightly spray hands with cooking spray if needed to prevent sticking.
Make a second batch in the same way using the remaining ingredients. Press into the second prepared pan. Let both pans cool at room temperature for 30 minutes.
Scoop a thin (1-inch) layer of ice cream over one pan of treats. Spread and press it into an even layer. (Any thicker of an ice cream layer will make it harder to eat.)
Lift the second block of treats out of the pan using the parchment paper. Flip it over and place it on top of the ice cream. Press down firmly and evenly, then peel off the parchment.
Cover tightly and freeze until solid, about 4-8 hours.
Use the parchment to lift the sandwich block onto a cutting board. Cut into bars using a very sharp knife that’s been run under hot water and dried. If the bars are too hard, let sit at room temperature for 5–10 minutes. Return any extras to the freezer promptly. Cover any leftovers tightly.
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Notes
Note1: Use soft, fresh marshmallows for the best bars. Don’t cook them too long or on high heat, or the bars will get too firm—especially after freezing.Note 2: Use plain Rice Krispies or try cocoa krispies or fruity pebbles for a fun twist. If you use cocoa krispies, chocolate ice cream is the best filling!Storage: Wrap each sandwich tightly in plastic wrap or foil. Store in the freezer and enjoy within a week for the best flavor and texture.