Can I substitute cornstarch for powder sugar in butter cream icing to make it less
sweet.I know is a weird question.

You could add a little salt, idk about cornstarch, never heard of anybody doing that.
There are several different recipes for buttercream and some are definitely sweeter than others,maybe you should try a different one.

Actually I add some cornstarch and powder dry milk to my buttercream
instead of some powder sugar.The buttercream was creamy not too sweet.
But I don't know if it is ok to do that.I have never heard of someone doing that.

i use icing mixture, this has a small percentage of corn starch in it. are you using pure icing sugar in your butter cream?

Yes I use powder sugar that has cornstarch in it.So I was thinking what will happen if I add more cornstarch and if I do how much?

Somebody else who what to tell me what do you think?

I, too, would be very interested in hearing what would happen if a person added a little more cornstarch and less powdered sugar.

I use a recipe that calls for a half cup of cornstarch along with the powdered sugar. It helps cut the sweetness and it helps for humid days.