In a medium saucepan over low heat, melt ½ cup of butter. Once melted, add the two packages of miniature marshmallows. Stir continuously until the marshmallows are completely melted and the mixture is smooth.
Remove the saucepan from the heat. Gradually add the 8 cups of Rice Krispies to the melted marshmallow mixture, stirring gently until all the cereal is evenly coated.
In another saucepan, combine the unwrapped caramels, the remaining ½ cup of butter, and the sweetened condensed milk. Heat over low to medium heat, stirring frequently until the caramels are completely melted and the mixture is smooth.
In your prepared 9x13 inch baking dish, spread half of the Rice Krispie mixture evenly across the bottom. Pour the melted caramel mixture over this layer, spreading it out evenly. Finally, top with the remaining Rice Krispie mixture, pressing down gently to create an even layer.
Allow the Caramel Rice Krispie Treats to cool at room temperature for about 30 minutes. For quicker setting, you can place them in the refrigerator for about 15 minutes. Once cooled, slice into squares and enjoy!