Place the dark chocolate and olive oil in a medium heatproof bowl. Place the bowl over a medium saucepan of simmering water. Stir until the chocolate is melted and smooth.

Place the butter and mini Mars bars in a medium saucepan. Stir over low heat for 5 minutes or until melted.

Place the rice bubbles in a large heatproof bowl. Add the melted Mars bar mixture and stir to combine.

Grease an 18cm x 28cm slice pan. Line the base and two long sides with baking paper, extending the paper 5cm above the edge of the pan. Press the mixture firmly and evenly into the prepared pan avoiding air pockets. Refrigerate until firm.

Spread the melted chocolate evenly over the slice. Refrigerate until set.

Using a hot knife, cut the slice into small bars.