Prep the pan: Line an 8x8-inch pan with parchment, leaving overhang for easy lifting.

Melt the base: In a medium saucepan over low heat, add chopped Mars bars, butter, and syrup. Stir slowly until smooth and glossy. Don’t boil.

Add flavor: Remove from heat. Stir in salt and vanilla if using.

Mix with cereal: Add the gluten-free crispy rice cereal to a large bowl. Pour the warm Mars mixture over it and fold gently until every piece is coated.

Press into pan: Transfer the mixture to the lined pan. Press down firmly with a spatula or slightly damp hands to create an even layer.

Top with chocolate: Melt the chocolate chips with the oil in short bursts in the microwave or over a double boiler. Spread evenly over the bars.

Set and slice: Chill for 45–60 minutes until the topping is set. Lift out using the parchment and cut into squares with a sharp knife.