Step 2

Chop the Mars bars and put them in a heatproof bowl with the butter, set over a pan of barely simmering water.

Step 3

Whisk as they melt until the caramel, nougat and butter come together into one smooth, glossy mixture with no lumps of nougat left.

Step 4

Pour the mixture over the puffed rice in a large bowl and fold until every grain is coated.

Step 5

Press it firmly and evenly into the pan with the back of a spoon, getting right into the corners.

Step 6

Melt the milk chocolate, either in short bursts in the microwave or over the same pan of simmering water.

Step 7

Spread the chocolate over the top and leave it somewhere cool to set, then lift the slab out and cut it into squares.