Prepare a cookie sheet with parchment paper. Set to the side.
In a large saucepan, melt the butter with the marshmallows. When the marshmallows look soupy, add the Rice Krispies treats. Mix the marshmallows and cereal together until fully combined.
Pour the cereal and marshmallow mixture onto the prepared sheet pan. Using a rubber spatula (spray it with cooking spray so the marshmallows won't stick to it), flatten the Rice Krispie treats on the cookie sheets. Let the cereal treats set at room temperature for 15 minutes.
While the Rice Krispie treats are setting, take the ice cream out of the freezer to thaw slightly. Then, turn the ice cream container on its side and slice it into round pieces. Each pint should yield about 3-4 slices of ice cream.
Assemble the ice cream sandwich. Use a round cookie cutter to cut a circle from the Rice Krispie treats cookie sheet. Top the treat with ice cream, followed by another Rice Krispie treat. Repeat until there are no more Rice Krispie treats left. Roll the ice cream in mini chocolate chips or sprinkles, if using.
