Step 1: Make the brandy butter on Christmas morning (it can be made the day ahead but it will need to be wrapped really well to avoid it becoming tainted in the refrigerator).
Step 2: Cream the butter and icing sugar in an electric mixer until white, thick and fluffy and the sugar has dissolved. This takes some time, so be patient.
Step 3: Slowly beat in the brandy, a teaspoonful at a time, tasting as you go. Cover with plastic film and refrigerate.