Line a 12-hole muffin tin with 12 large cupcake cases. Preheat the oven to 180ºC, fan 160ºC, gas 4. Place 150g butter plus the sugar, eggs and milk in a large bowl and mix with an electric whisk to combine.
Sift the self-raising flour, cinnamon, nutmeg and mace into the bowl, then fold through the butter and sugar mixture.
Divide the mixture between the paper cupcake cases, then bake for 25 minutes until cooked through. Remove from the oven and allow to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.
To decorate, mix 140g butter with the icing sugar until smooth. Spoon the icing into a piping bag fitted with a star-shaped nozzle. Pipe a swirl on the top of each cake. Decorate with holly and gold pearl cake decorations.