Preheat oven to 180 C / Gas 4. Grease and flour a 23cm tube cake tin.
In a large bowl, cream together the butter and caster sugar until smooth. Beat in the eggs one at a time, mixing well after each, then stir in the vanilla. Combine the flour, bicarbonate of soda, salt and mace; mix into the butter mixture just until smooth. Stir in soured cream. Spoon cake mixture into the prepared tin and spread evenly.
Bake for 1 hour and 20 minutes in the preheated oven. Cool for at least 10 minutes before inverting the cake tin onto a plate and tapping out the cake. Dust with icing sugar before serving.