The finished sauce should be refrigerated before use, so make it the day before because there won’t be time on Christmas Day morning. Find a suitable rectangular glass or metal dish (a small casserole dish will do) and put it in the fridge to cool down whilst you mix the sauce.
Alternatively, some people roll the mixture up in a piece of plastic film wrap.
Beat the butter in a food mixer until it is pale and creamy. Add the icing sugar and beat until it is light and fluffy. Gradually add the brandy. Put the mixture in the chilled dish, or roll up in plastic film to make a long sausage, and put it in the fridge for at least two hours, over night preferably. Cut into neat cubes or similar shapes for serving, and in our hot climate, keep it chilled until the last minute!
When I helped my mother make this, the brandy was always “slightly more”, with gradual additions until the mixture just started to look as if it would break up, when it was into the fridge as quickly as possible!