I've seen a pistachio buttercream on Martha Stewart's Wedding Cakes. It calls for pistachio paste which is very hard to find. I did find some while on vacation in LA at Surfas. It still sits in my pantry (haven't had a chance to make it yet). Hopefully, I can try it out this summer now that school is out.
there is a pistachio pudding mix. maybe you could use that and make a mousse.
2 cups liquid Rich's bettercreme
1 cup milk
1 pkg. instant pistaschio pudding mix
(optional- extra crushed pistaschios...there are some in the pudding)
whip it all together and enjoy....it's delicious!!!
You could also just whip a pkg. of the pudding mix with a tub of cool whip.......however, by doing it with the Rich's bettercreme it is safe at room temp for up to 5 days!! Which is a good thing with wedding cakes!
Not only is pistachio paste hard to find, but Wow! The price! 
I gave up looking for pistachio paste. I love the idea of pistachio pudding mix. I'm going to give that a try.
Then again, what about grinding up some pistachios and adding them to a buttercream icing?
Definitely the pudding idea sounds better and better. 
I just had another idea. Instead of adding Dream Whip to Indydeb's recipe, you could add 3 Tblsp of pistachio pudding mix instead. Maybe Indydeb will chime in and say what she thinks of the idea.
You can find a can of pistachio paste here: www.pastrychef.com it is pricey, but good inredients make a difference. I made Martha's pistachio bc and it was great!
That's where I found it!
But it is $72 for something like a pound. And I only needed 3 oz. But then again I guess I could just keep using it about 5 more times. 
Someday I'll take the plunge and get some. Then I'll be hooked and have to have it all the time. Sigh......
You can find a can of pistachio paste here: www.pastrychef.com it is pricey, but good inredients make a difference. I made Martha's pistachio bc and it was great!
That's where I found it!
But it is $72 for something like a pound. And I only needed 3 oz. But then again I guess I could just keep using it about 5 more times.
Someday I'll take the plunge and get some. Then I'll be hooked and have to have it all the time. Sigh......
I know, I only wanted a small amount too, oh well
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