Set the oven temperature to 175°C, fan. Grease a rectangular, loose-bottomed cake tin, approx. 13 x 36 cm.
Chop up the chocolate and set it aside. Melt the butter and stir in the chocolate. Stir until the chocolate has melted, then add the sugar and stir to a paste.
Stir in the egg and flour. Pour the mixture into the tin and bake at the centre of the oven for about 5 minutes. Allow to cool.
Make the marshmallow filling. Heat the marshmallow fluff with the peanut butter in a saucepan and mix them together. Spread the filling over the mud cake and refrigerate until set.
Make the toffee. Chop up the peanuts and set aside. Combine the other ingredients in a heavy-based saucepan. Boil until the temperature is 120°C or test the toffee by dropping a little of the mixture into a glass of cold water. If it makes a round, soft ball, the toffee is ready. Add the peanuts. Distribute the toffee evenly over the marshmallow filling and refrigerate for about an hour until set.
Chop and melt the dark and milk chocolate for the topping over a water bath and spread the chocolate evenly over the toffee sauce. Sprinkle with chopped peanuts. Refrigerate until set, then cut into pieces.
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