Lightly grease a 6-inch-by-2½-inch cake ring with butter and place it on a tray lined with parchment paper.
Break each biscuit into almond-sized pieces by hand and set them aside.
In a large bowl, mix the softened butter and sugar until it lightens.
Melt the 4 ounces of dark chocolate and add it to the butter mixture, stirring constantly.
Add the egg and beat to combine.
Gently fold in the biscuit pieces until they are all coated with the chocolate mixture.
Spoon the mixture into the prepared cake ring, ensuring it fills all the gaps on the bottom (which will be the top when unmolded).
Chill the cake in the refrigerator for at least 3 hours.
Remove the cake from the refrigerator and let it stand.
Meanwhile, melt the 8 ounces of dark chocolate in a double boiler or saucepan on the stove top over low heat. Slide the ring off the cake and turn it upside down onto a cake wire.
Pour the melted chocolate over the cake, smoothing the top and sides with 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 it to the cake wire, and lift it onto a tea plate.
Melt the remaining 1 ounce of chocolate and use it to decorate the top of the cake. Enjoy your royal treat!