Getting rid of a commercial/plastic taste in icing?
The ingredients they post are:


1 cup solid vegetable shortening
1 teaspoon Wilton Flavor (vanilla, almond or butter)
7-8 teaspoons milk or water
1 lb. confectioners' sugar
1 tablespoon Meringue Powder
pinch of salt (optional)


I would make the following suggestions based on these ingredients:


It's not really butter cream icing if you're using vegetable shortening.  You'll get far superior flavour by using butter.  Pure butter may have trouble holding its shape, but you should have no trouble at all switching to a half-and-half mixture.  This ratio won't change the colour much either.
I've never personally cared for the Wilton flavourings and suspect that is a major part of the problem here.  If you're using the vanilla, it's not real vanilla, it's imitation vanilla extract; pure vanilla extract is dark brown and would colour the icing.  Their almond flavour also seems to taste nothing like normal almond extract, and the butter flavour is thoroughly pointless (just use butter!).

You can use real vanilla extract, but it will definitely darken the icing.  You can try to compensate with an icing whitener, but those things are intended for slightly-off-white icings and probably won't get you all the way to white.  Alternatively, you can use pure almond extract instead of the imitation; it is actually clear to begin with.

Or, better yet, you can just change your brand.  Lorann makes a bunch of concentrated flavouring oils that might taste less fake.  If you go to any baking supply store you'll probably see other brands as well.  Try a few until you find one that you like.
You don't say whether you're using milk or water, but use whole milk or even cream if you can; don't miss an opportunity to lay on the fat (and therefore flavour).
Finally, a lot of people (me, for example) find the taste of meringue powder to be disgusting.  You don't need it, and the vast majority of buttercream icing recipes don't contain it.

If you really want a meringue-based buttercream, as opposed to a simple buttercream without any egg products at all, then just go for broke and make it from an Italian meringue; whip some egg whites to soft peaks, then whip boiling sugar syrup into the eggs until the mixture stiffens, and afterward incorporate the butter and other flavourings.

I know, they're going to tell you that it won't be stiff enough.  But stabilizing a meringue is dead easy; just bloom some gelatin in the cold liquid (water/milk/cream) before you incorporate it1.  The more gelatin, the stiffer it will get.  Don't overdo it because if you add too much gelatin, you won't even be able to get it out of the bag (this has happened to me). A 1% ratio should be plenty.


Making a great-tasting decorating icing is easy.  Stiffness isn't usually the problem, colour is.  If you want brilliant white icing then you have to make certain tradeoffs.  If you're willing to settle for a cream colour (or slightly whiter, with a whitener) then you've got nothing to worry about.



1. If you're not starting with a hot meringue, then be sure to heat the liquid with bloomed gelatin to dissolve before adding it to the icing!