Lightly butter a 9x13-inch pan using 1 tablespoon of the 5 tablespoons of butter from the ingredient list; set the pan aside. Measure 6 cups crispy rice cereal into a large mixing bowl.

In a large saucepan, melt the remaining 4 tablespoons of butter over medium-low heat.

As soon as the butter is melted, add 4 heaping cups mini marshmallows to the saucepan. Stir constantly until the marshmallows are fully melted and the mixture is smooth. Remove from heat.

Pour the melted marshmallow mixture over the measured cereal. Gently stir with a spatula until the cereal is evenly coated.

Transfer the cereal-and-marshmallow mixture to the prepared 9x13 pan. Press it into an even layer, pressing gently so the treats remain light and retain air pockets. Refrigerate the pan while you make the caramel.

For the caramel, combine in a medium saucepan: 1/2 cup butter, 1/2 cup light brown sugar, 1/2 cup light corn syrup, half of the 7-ounce can sweetened condensed milk (the 3.5-ounce portion), and a dash of salt. Cook over medium heat, stirring until the sugar dissolves.

Bring the caramel mixture to a rolling boil, then continue to cook, stirring constantly, until it reaches the soft-ball stage (about 235–240°F on a candy thermometer). If you don’t use a thermometer, use the ice-water test: drop a small spoonful into very cold water; it should form a soft, pliable ball that flattens when removed. Remove the caramel from heat as soon as it reaches the soft-ball stage.

Immediately pour the warm caramel in an even layer over the chilled rice krispie base. Use an offset spatula or the back of a spoon to smooth the caramel. Allow the caramel to set at room temperature for at least 30 minutes, or place the pan in the refrigerator until firm.

To make the chocolate topping, place 2 cups milk chocolate chips and 1 tablespoon butter in a heatproof bowl. Melt using a double boiler or in the microwave in 30-second intervals, stirring between intervals, until the chocolate is completely smooth.

Pour the melted chocolate over the set caramel and spread into an even layer. Refrigerate until the chocolate is fully set.

Once set, cut into bars. Store the bars in the refrigerator; they will keep for at least a week.