Preheat the oven to 180ºC/gas mark 4. Line a 12-hole cupcake tray with cupcake cases. In a large mixing bowl, combine the flour, cocoa powder, baking powder and sugar. In a large jug, beat together the oil, buttermilk, vanilla extract, red food colouring and egg.
Whisk the wet ingredients into the dry ingredients until the mixture is smooth. Spoon the mixture into the paper cases. Bake for 20 minutes until a skewer inserted into the centre comes out clean, then transfer to a wire rack and leave to cool completely.
Beat the butter until smooth, then beat in the soft cheese, icing sugar and vanilla extract until combined. Put half the mixture into a small bowl, add the red food colouring and beat until evenly coloured.
Spoon the red icing into one piping bag, and the white into another. Snip a 1cm hole in each, then place both bags into the third bag fitted with the star-shaped nozzle. Pipe on to the cooled cupcakes, creating a two-tone cream cheese frosting.