Prepare the Pan: Line an 8×8-inch or 9×9-inch baking pan with parchment paper. Set aside.
Make the Base: In a saucepan over medium heat, melt the Mars Bars and butter together. Stir occasionally until smooth. Once melted, add the Rice Krispies and mix to coat the cereal evenly.
Press into the Pan: Transfer the mixture into the prepared pan, pressing it down firmly to form an even base.
Prepare the Topping: In a microwave-safe bowl, melt the chocolate chips and butter together. Stir every 30 seconds until smooth and fully melted.
Coat the Bars: Pour the melted chocolate over the Rice Krispies base and spread it evenly.
Set and Slice: Place the pan in the fridge for at least 1 hour to set the chocolate. Once firm, slice into squares and serve.
Store: Store in an airtight container at room temperature for up to 4 days, or refrigerate for longer storage.
