Make cake according to directions on packet. Cook in a square tin. Cool.
Level top of cake and cut in half. Lay halves side by side.
Make the ganache: melt the cooking chocolate, 1/4 cup cream and butter in a heatproof bowl over a pot of boiling water, stirring till smooth. Cool slightly then spread evenly over both cake halves.
Whip both 600ml bottles of cream with the peppermint essence and food colouring.
Sandwich each of the biscuits together with a heaped teaspoon of the cream and lay across the width of the cake. Continue till the cake is completely covered.
Use the rest of the cream to cover the top and sides of the biscuits and cake.
Cover and refrigerate for at least 12 hours. Overnight is better but if you need it in a hurry, you can quickly dip each of the biscuits in milk before you sandwich them.
Decorate with crushed peppermint crisps and mint thins.
To serve, cut across the width of the cake so as to get the stripes.
Tip
This is a big cake (cos there's lots of us) so just halve the recipe for a smaller version.