Preheat the oven to 170 °C, (165°C for a fan oven), or Gas Mark 5. Generously grease 3×8 inch cake tins and line with greaseproof paper.
Mix the chocolate cake mix, eggs, oil and water in a large bowl on low speed for 1 minute until moistened. Mix on medium speed for a further 2 minutes.
Pour equal amounts of mixture into the three prepared cake tins.
Bake in the oven for 20-25 minutes until cooked. To check if cooked, insert a skewer into the centre of the cake. If skewer comes out clean then the cake is ready.
Leave to cool before removing from the tins.
Reserve 5 biscuits for the top, blitz the rest to powder, not too fine.
Mix 3/4 of your powder with soft butter and icing sugar to make speckled frosting and filling.
Spread as filling in-between each layer and sandwich and cover the cake, use spare powder to decorate base of cake and plate.
Cut 5 biscuits in half to decorate top, sit in piped dot of icing to hold in place.