1 - Grease and line a 20 x 20cm/8" x 8" baking tin with parchment paper.

2 - Put the mars bars and butter in a heat proof bowl and place over a pan of barely simmering water.

3 - Mixing with a whisk, melt until the mixture is smooth.

4 - Pour over the rice krispies in a mixing bowl and mix until all the ingredients are evenly combined.

5 - Press evenly into the prepare baking tin and set aside.