Step 2

Put the Mars bars and butter in a heatproof bowl set over a pan of barely simmering water. Melt together, whisking until smooth.

Step 3

Pour the mixture over the Rice Krispies in a large bowl and stir until evenly coated. Press firmly and evenly into the prepared tin.

Step 4

Melt the milk chocolate in the microwave or over a pan of barely simmering water.

Step 5

Spread the melted chocolate over the crispy layer and leave to set in a cool place. Once set, slice into squares.