In a large saucepan, melt the butter over low heat. Add the marshmallows and stir until completely melted.

Remove from heat and quickly stir in the Rice Krispies cereal until they are well coated with the marshmallow mixture.

Using a buttered spatula or wax paper, press the mixture into a 13x9 inch pan coated with cooking spray. Set aside to cool.

In a medium saucepan over low heat, combine the soft caramels and heavy cream (or evaporated milk). Stir continuously until the caramels are fully melted and the mixture is smooth.

Pour the caramel over the prepared Rice Krispies layer, spreading evenly with a spatula. Allow to cool and set. If using, place the pan in the refrigerator for about 30 minutes to help the caramel layer firm up faster.

If desired, melt the chocolate chips in the microwave in 30-second intervals, stirring in between, until smooth. Drizzle the melted chocolate over the caramel layer with a spoon or a piping bag.

Once the caramel (and chocolate, if used) has set, cut into squares or bars.

Serve at room temperature for easier eating, as the caramel can harden when chilled.