Cut mars bars into cubes a put into a glass bowl. Add syrup and butter.
Place bowl over a bain marie (pan 1/4 filled with already boiled water). Ensure the water does not touch the bottom of the bowl.
Gently melt the chocolate, stirring carefully as it does so to combine the ingredients.
Once melted fully, stir in a spoonful of rice crispies at a time. Carefully fold into the chocolate mixture and repeat.
Press gently into a lined tin. Use a smaller tin for deeper slices.
Pop into the fridge to set for at least 30 minutes.
Slice into bitesize squares using a very sharp knife.
