Line a baking dish (about 20x20cm) with baking paper.
Break up biscuits into small pieces, about 2cm.
Place butter and chocolate mix in a large pot on low heat. Heat for about 2 minutes, until butter has melted. Whisk in egg and increase heat to high. Bring to a boil then remove from heat. Add broken biscuits and stir to combine.
Press slice mix into prepared dish and place in the fridge for 1 hour to set. Slice into squares and dig in!