Line an 18 x 27 cm slice tin with baking paper to prevent sticking.

In a microwave-safe bowl, combine the dark chocolate, condensed milk, and butter. Heat in 30-second bursts, stirring after each until completely melted and smooth.

Pour the melted chocolate mixture into a large bowl. Add the finely crushed biscuits and rice bubbles, mixing until everything is evenly combined. Press into the lined tin, smoothing the top with a spatula.

For the topping, melt the remaining dark chocolate with vegetable oil in the microwave, stirring until smooth. Add peppermint essence and adjust with more oil if it feels too thick.

Pour the topping over the slice base, spreading it evenly. Chill in the fridge for at least 2 hours until set. Slice into pieces and serve.