What did I do wrong? It's great for crumb coating, but as the top frosting it doesn't slice well--kinda chunks off.

Surprisingly, this is a recipe I use all the time and have only had this happen a couple of times. Any suggestions? Not enough shortening and too much sugar?

Yeah, sounds too stiff to me. Try adding in another teaspoon of water and see if that helps, adding very little water at a time. It cant hurt. Ive used pretty creamy frosting before, thinking it may be too thin, but I found out that it just takes a little bit longer to crust, thats all. Better that, than too stiff I think. Every time I make frposting, the amount ofliquid I use changes. I think it depends on your house, temps, humidity, etc. Sometimes I ise 1/2 cup of milk or water to a batch, and sometimes I have to use a LOT more. I to the finger test, iafter its mixed, I stick my finger in and press down on the icing, if its firm the touch and is not sticky (and just leaves an indent), then its too stiff. If its firm, but slightly sticky, its perfect.

Crusting is determined by the ratio of sugar to fat. Less fat more crusting so I would add a little more fat (shortening).