Step 1: Grease a 9x13 inch baking dish with cooking spray or butter, then line with parchment paper, leaving an overhang on the sides. Lightly spray the parchment paper as well.
Step 2: In a large, heavy-bottomed pot, melt the butter over medium-low heat. Add the marshmallows and stir constantly until they are mostly melted (about 75-80%). Remove from heat and continue stirring until completely smooth. Stir in the vanilla extract.
Step 3: Add the Rice Krispies cereal to the pot with the marshmallow mixture. Gently fold until the cereal is evenly coated.
Step 4: Press about two-thirds of the Rice Krispies mixture evenly into the prepared baking dish. You can use a buttered spatula or your buttered hands for this. Set aside.
Step 5: In a microwave-safe bowl, combine the unwrapped caramels and 2-3 tablespoons of milk or cream. Microwave on high in 30-second intervals, stirring after each, until the caramels are melted and smooth. Alternatively, melt in a small saucepan over low heat, stirring frequently.
Step 6: Drizzle about half of the melted caramel evenly over the Rice Krispies base layer in the pan. Spread gently with an offset spatula if needed.
Step 7: Gently crumble the remaining one-third of the Rice Krispies mixture over the caramel layer in the pan. Then, drizzle the remaining melted caramel over this second layer.
Step 8: In a separate microwave-safe bowl, melt the chocolate chips in 30-second intervals, stirring until smooth. Drizzle the melted chocolate generously over the top of the caramel chocolate rice krispies. If desired, sprinkle with a pinch of flaky sea salt.
Step 9: Allow the caramel chocolate rice krispies to cool completely at room temperature for about 1 hour, or in the refrigerator for 30 minutes, until the caramel and chocolate are set. Once set, use the parchment paper overhang to lift the entire slab out of the pan. Cut into squares and serve.