Line a 18 X 27cm slice tin with baking paper and set aside.
Add chocolate, condensed milk and butter to a microwave-safe bowl. Heat in 30 second bursts until all the ingredients have melted together and stir well.
Add to the crushed biscuits and rice bubbles and mix until all combined. Press the mixture into the tray, spread to the edges and smooth the top with the back of a metal spoon.
For the topping: Melt together the rest of the dark chocolate with the oil in the microwave. Stir until it is smooth then add the peppermint essence and stir again. If it seems a bit thick then add more oil until it is nice and runny.
Pour the chocolate over the base and spread it evenly to the edges. Chill in the fridge until set then use a large knife to cut into slices and serve.
