Step 3 - In a large bowl, combine the butter and sugar until the mixture starts to lighten
Step 4 - Melt the 4oz of dark chocolate and add to the butter and sugar mixture, stirring constantly
Step 5 - Add the egg, and beat to combine
Step 6 - Fold in the biscuit pieces until they are all coated with the chocolate mixture
Step 7 - Spoon the mixture into the prepared cake ring, making sure you fill all the gaps in the bottom
Step 8 - Chill the cake in the fridge for at least 3 hours