Line a 11x7inch (27.9x19.8cm) inch baking tray with some greaseproof paper and set aside.
In a large bowl, melt the butter/margarine and Mars Bars over a bain marie, or in short bursts in a microwave.
Add the rice krispies to the melted ingredients and mix well, ensuring all are coated.
Tip into your prepared baking tray and push down with the back of a spoon, ensuring an even finish right into the corners. Set aside.
Melt the milk chocolate over a bain marie, or in short bursts in the microwave, before spreading over the top of your rice crispy base.
Place into the fridge to set for at least a few hours or ideally overnight. Cut into your desired portion sizes to serve.