Line a 12-section muffin tray with paper muffin cases. Combine the flour, cocoa powder and bicarbonate of soda in a bowl. Mix together the buttermilk and vinegar in a jug.
Beat together the butter and caster sugar in a separate bowl until pale and creamy, then beat in the egg and beetroot.
Sift half the flour mixture into the bowl and stir in gently with a large metal spoon. Stir in half the buttermilk mixture. Sift and stir in the remaining flour mixture, then the remaining liquid. Divide the muffin mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Beat the cream cheese with a wooden spoon in a bowl until softened. Beat in the vanilla extract and icing sugar until smooth. Swirl over the tops of the cakes and decorate each with a cherry.