You will need:
60g unsalted butter at room temperature
150g caster sugar
1 egg
20g cocoa powder
40ml red food colouring
½ tsp vanilla extract
120ml buttermilk
150g plain flour
½ tsp bicarbonate of soda
1½ tsp white vinegar
 
For the frosting:
300g icing sugar, sifted
50g unsalted butter, at room temperature
125g cream cheese, cold