I have had this cake before and I am posting it in celebration of Prince William's wedding. If you can't find McVitie's Tea Biscuits, you can use another brand of rich tea biscuits. I use a chocolate ganache to top off the cake. You can also double this recipe to get height in the cake. What I have also heard is that this is a cake often shared between Queen Elizabeth and Prince William when they would get together for tea. He loves it so much he is having it as his groom's cake.
Lightly grease a small (such as 6 x 21/2-inch) cake ring with 1/2 teaspoon butter, and place on a parchment-lined tray. 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 is a light lemon color.
Melt the 4 ounces of dark chocolate in a double boiler. Add the butter and sugar mixture to the chocolate, stirring constantly. Add the egg and continue stirring. Fold in the biscuit pieces until they are all coated with the chocolate mixture.
Spoon the chocolate biscuit 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 three hours.
Remove cake from cake pan and turn upside down onto a cookie cooling rack. In a small bowl place remaining 2/3 cup chocolate chips. In 1-quart saucepan, heat remaining 1/3 cup whipping cream and 1 tablespoon butter over medium heat, stirring until butter is melted and mixture comes to a boil. Pour cream mixture over chocolate and stir smooth. Pour the melted chocolate over the cake; frost top and sides using a butter knife or offset spatula. Allow the chocolate ganache to set at room temperature. Carefully run a knife around the bottom of the cake where it has stuck to the cooling rack, and transfer the cake to a cake dish.
If desired, decorate with melted milk or white chocolate in a decorative pattern.