I assume by sifting "buttercream" you mean you didn't sift the powdered (icing) sugar. You could try putting the buttercream through a fine seive, but I really think you should start over.
Ganache: just substitute white chocolate for the brown chocolate.
"Not the plastic but the fondant" - I don't understand what you are saying. What part of ganache is plastic?
pastrypet, Sorry I did mean powder sugar. chocolate plastic is basically white choc. and corn syrup. you can roll it out to cover cakes. I was looking for a marshmallow fondant recipe with white choc in it. I'll be covering a wedding cake .
I never sift my powdered sugar. Just use a paddle attachment and whip it on high. The paddle is meant to press things against the side of the bowl and smooth them out.
Long slow beating should help. I have a recipe that takes up to 30 minutes and it is the creamiest and I *NEVER!* sift.
Just put it back in the mixer bowl, set it on low and go watch the news or read the paper :) Check it after 15 minutes.
Kakeladi please share that recipe I've never heard anything like it!
It is posted in the frosting section of the recipe section:) It's called "2 of everything".
Sorry I can't give you a link right now.