Preheat the oven to 170°C fan oven/190°C electric oven. Butter two (or three if you have them) 20cm cake tins with removable bases, 5cm deep, and line the bases with baking paper.
To make the cake, cream the butter and sugar together for 3–4 minutes in a food processor until really light and fluffy. Add the eggs one at a time, scraping down the sides of the bowl between each addition. Add the flour in three batches, alternating it with the buttermilk so you start and end with the flour. Next mix in the salt, vanilla and cocoa. Start to add the red food colouring, a knife-tip at a time, until the mixture is a dramatic dusky red – I find it needs about ½ teaspoon in all. Mix the vinegar and bicarbonate of soda together in a small bowl, they will fizz, and add to the batter. If using three cake tins,* divide the mixture between them, weighing them for accuracy to ensure you get evenly thick sponges at the end. Bake for 20–25 minutes or until shrinking from the sides and firm when pressed in the centre. Leave to cool in the tins for 10 minutes, then turn the cakes out onto a wire rack, remove the paper and leave to cool top upwards.
To make the frosting, place the butter and icing sugar in the bowl of a food processor and cream together. Transfer to a bowl and blend in the cream cheese and vanilla extract, beating with a wooden spoon until smooth.
Use about one-third of the buttercream to sandwich the three layers and the rest to coat the top and sides. Place the finished cake on a plate or board, cover with clingfilm and chill for 1 hour until set. If chilling it any longer than this, remove from the fridge 30 minutes before serving. It keeps well in an airtight container for several days.
*If using two tins, add a third of the mixture to each one, bake these, and then bake the third sponge while the other two are cooling.
TIP: I find food colour paste much more effective than liquid for dyeing the sponge a rich vermillion red.
Extract from Annie Bell’s Baking Bible, by Annie Bell, published by Jacana.