I got the idea for this frosting many years ago, when I read that, on her 100th birthday, the Queen Mum's cake had a gin frosting. I don't think I'd reached the legal drinking age yet, but I already knew that sounded delicious! This is NOT a recipe for a child's birthday cake! It's for a very grown-up birthday cake to get your grown-up friends tipsy on cake. Not for those under the legal drinking age in your jurisdiction.
I finally found the perfect frosting to absorb the gin without becoming runny - Swiss meringue. Although it sounds complicated, it's actually a very forgiving recipe. In fact, one of the best things you can do is ignore it for several minutes at a time. You just need a stand mixer and a candy or meat thermometer that can read 160 degrees Fahrenheit.
This makes enough to frost a 9-inch, 2-layer cake. If you don't want to use raw eggs, or have no use for a lot of yolks, pasteurized egg whites from a carton work equally well.