This is such an old kitchen trick that I feel that there’s little point to mention it, but I will anyway. You can make an excellent icing with icing/confectioner’s sugar and cold coffee. It depends on how big your cake is, but start with ¼ cup of icing/confectioner’s sugar and add 2 teaspoons of cold coffee. Mix thoroughly and check the consistency. If you want your icing thicker, then add more sugar. If you want it thinner, then add a very small amount of coffee. If you want more icing, then add more sugar and coffee. The trick is to add as little liquid as possible, so that you aren’t constantly going back and forth between the sugar and coffee.
I use this icing mostly on brownies, but don’t let that stop you from using your imagination. I think this would also make a good filling for sandwiched cookies or as icing on a sugar cookies.