Line a rectangular dish or baking tin with vegetable oil or coconut oil and parchment paper.
In a small saucepan set over medium-low heat, melt the butter and 200g chopped mars bars together. Stir occasionally to ensure the two are well mixed, once melted add the salt and vanilla then stir to combine.
Working quickly, add the rice krispies to a large bowl and pour the melted chocolate and butter mixture over the rice cereal and stir with a rubber spatula until the rice cereal is well coated.
Pour the rice and mars bar mixture into the prepared tin, and use a spatula or the back of a metal spoon to press the mixture into the dish to create a flat even layer.
Place the chopped chocolate into a heatproof bowl and melt the chocolate in the microwave in increments of 30 seconds. Stir between each increment to ensure the chocolate is melting evenly.
Once the chocolate has melted, pour it over the pressed rice cereal. Use a knife or offset spatula to smooth the chocolate over the surface of the mars bar slice. Top with the extra sliced Mars Bar pieces.
Place the slice in the fridge for about 2 hours to firm up. Remove the Mars Bar slice from the fridge and use a sharp knife heated in hot water to slice it into 5cm (2-inch) square pieces. Serve and enjoy!