I made buttercream using only butter and pwd sugar and cocoa pwd plus water but it is not crusting and is soft . I know I can add more pwd sugar but I don't want it to be too sweet. Is there any other ingredient I can add to stiffen it?

You could try adding a couple tablespoons of meringue powder.

HTH

Crusting is about the ratio of powdered sugar to fat. If you need it to be stiffer and it is not crusting the way you want, then it doesn't have enough powdered sugar in it.

Unfortunately, powdered sugar based icings ARE extremely sweet. That's just the way they are. If you are finding them too sweet for your taste, you may want to try IMBC, SMBC, or FBC. However, none of those crust, so if the crusting is vital to you, then you'll have to stick with the powdered sugar icings.

What were the amounts of the items you used?

thank you all for your help.