Preheat the oven and butter the tins. Preheat the oven to 180°C/160°C Fan/Gas 4. Grease and line the two sandwich tin bases of 20cm (8in) with non-stick baking paper.
Mix the cake's dry ingredients. Mix thoroughly the flour, cocoa powder, baking powder, bicarbonate of soda and sugar in a bowl.
Mix the cake's wet ingredients. Add the buttermilk, oil, vanilla, food colouring and 100ml (3½fl oz) water in a jug. Add the eggs and whisk until smooth.
Mix dry and wet ingredients. Pour and whisk the wet ingredients into the dry ingredients until combined into a bright red mixture. You can also add more colouring if you think it's not bright enough. Note that normally the mixture will get darker as it cooks
Bake the two cakes. Divide the mixture evenly between the two prepared tins levelling the surfaces and bake for about 25–30 minutes. When baked the cakes should be well risen and shrinking away from the sides of the tins. Leave to cool in the baking tins for around 10 minutes and then leave to cool completely on a wire rack.
To make the buttercream icing. Place the soft butter in a large bowl adding the vanilla extract and sift in half the icing sugar. Mix with an electric whisk until smooth. Sift in the remaining half of the icing sugar whisking well. Finally, add the mascarpone cheese to the mixture and gently stir with a spatula (not the whisk) until smooth. Note that beating with a whisk may cause the buttercream to split. Put a fluted nozzle in a piping bag and fill in about 2/3 cup (150g/5oz) of the buttercream into the bag.
To assemble the cake and chill. On a cake plate place one of the sponges spreading a third of the buttercream on top. Then using the remaining sponge cover with the other sponge, closing the sandwich cake. Ice the cake with a "crumb coat" by spreading a thin layer of icing and chilling for 30 minutes.
Decorate & chill. Pile the remaining icing from the bowl on top and spread over the top and the edges. Make sure that the icing is smooth around the edges before covering the sides. Then with a small palette knife make wide lines up the sides and swirl the top. With the fluted nozzle, pipe a rope design around the top edges of the cake and decorate nicely with the 8 chocolate truffles to finish and chill again before serving.