A tender, tangy buttermilk sponge is the foundation for this cake’s striking red interior. The colour can depend on the cocoa you choose. Dutch process cocoa, which is washed with a potassium carbonate solution to neutralise its acidity, produces a darker, more red cake; natural cocoa is lighter in colour. Flavourwise, though, you’ll be met with the same deliciousness.
Make the sponges. Heat the oven to 200°C/180°C fan/400°F/Gas 6. Beat the butter and sugar in a stand mixer fitted with the beater, on medium speed for 3–5 minutes, scraping down the inside of the bowl from time to time, until pale and creamy. Add the eggs, a little at a time, beating well between each addition until combined.
In a small jug, mix the cocoa powder, vanilla, red food colouring and hot water together to form a paste. Add this to the bowl and mix well until combined.
Combine the buttermilk and salt in a jug. Mix one third into the bowl, then add one third of the flour. Continue to mix in the buttermilk and flour alternately until thoroughly combined.
In a small bowl, mix the vinegar and bicarbonate of soda together, then add this to the bowl and beat gently until the mixture is smooth.
Divide the mixture equally between the lined tins and spread it level. Bake the sponges on the middle shelves for 20–25 minutes, until risen and a skewer inserted into the centres comes out clean. Leave the sponges to cool in the tins for 5 minutes, then carefully turn them out onto a wire rack to cool completely.
Make the filling and icing. Whisk the cream cheese, cream, vanilla, icing sugar, mascarpone and salt in a stand mixer fitted with the whisk until the mixture is smooth and holds stiff peaks. Divide the icing equally between two bowls, then chill it until needed.
Assemble the cake. Remove one bowl of the icing from the fridge. Using a large, serrated knife, level the sponges, keeping any offcuts for the decoration. Slice each sponge in half horizontally so you have 6 thin sponges. Smear a little of the icing onto the middle of a cake plate or cake stand and top with the first sponge.
Spread one fifth of the icing over the first sponge, then top with a second sponge. Spread with another fifth of the icing, then top with a third sponge. Continue layering until you have 5 layers of icing between 6 sponges.
Remove the second bowl of icing from the fridge. Using a palette knife, spread half of the icing over the outside of the cake to seal and neaten. Clean the palette knife and smooth off any excess icing to create a smooth even icing layer.
Using half of the remaining icing, spread a second thin, even layer over the top of the cake and smooth it with a palette knife.
Spoon the remaining icing into the piping bag fitted with a medium closed star nozzle and pipe 8 rosettes around the top edge of the cake.
Crumble the reserved offcuts of sponge and gently press these up the bottom third of the cake. Sprinkle a few crumbs over each piped rosette to finish.