If your coffee liqueur is on the sweet side, then add six drops of vanilla bitters to help the balance and complexity. However, with a good coffee liqueur...
I don't know if I should be pleased or disappointed that such a simple recipe worked so well. I tried this with tequila instead of gin as another commenter mentioned here, and while it was essentially a different drink, it was still very good. These simple 2-ingredient recipes are so annoying when you spend ages making a new, 6-ingredient mix that will please your friends just as much as this one will. Oh well.