Prepare the Mars bars and butter:

Chop the Mars bars into small pieces.

In a medium saucepan, melt the butter and Mars Bar pieces over low heat. Stir continuously until the mixture is smooth and well combined.

Mix with rice bubbles:

Once melted, remove the saucepan from heat.

Stir the rice bubbles quickly until the Mars Bar mixture coats them evenly.

Press into a tin:

Line a 20-cm-square baking tin with parchment paper.

Press the rice bubble mixture firmly and evenly into the tin, ensuring it is level.

Melt the chocolate.

In a microwave or using a double boiler, melt the milk chocolate until smooth.

Pour the melted chocolate over the rice bubble mixture, spreading it evenly to cover the entire surface.

Set and Slice:

Place the tin in the refrigerator for about an hour, or until the slice has set.

Once set, remove the slice from the tin, cut it into squares, and serve.