Servings: Makes one 6-inch round cake (8 portions)
To make cake: Lightly grease a 6" x 2 1/2" cake ring and place on a tray on a sheet of parchment paper.
Break each of the biscuits into almond-size pieces by hand and set aside.
Cream the butter and sugar in a bowl until the mixture starts to lighten.
Melt the 4 ounces of dark chocolate and add to the butter mixture while constantly stirring.
Beat the egg into the mixture.
Fold in the biscuit pieces until they are all coated with the chocolate mixture.
Spoon the mixture into the prepared cake ring. Try to fill all of the gaps on the bottom of the ring because this will be the top when it is unmolded.
Chill the cake in the refrigerator for at least 3 hours.
To coat and decorate: Remove the cake from the refrigerator and let it stand while you melt the 8 ounces of dark chocolate.
Slide the ring off the cake and turn it upside down onto a cake wire.
Pour the melted chocolate over the cake and smooth the top and sides using a palette knife.
Allow the chocolate to set at room temperature.
Carefully run a knife around the bottom of the cake where the chocolate has stuck to the cake wire and lift it onto a tea plate.
Melt the remaining 1 ounce of chocolate and use to decorate the top of the cake.