I remember when I was little my Mom used to decorate wedding cakes. Her frosting recipe was as easy as they come. To save money and make the cake most shelf-stable, she would recommend that her clients have "butter creme" frosting, instead of butter frosting.

Basic butter frosting:
-Whip butter. Add powdered sugar to desired consistency.

Basic Butter Creme frosting:
-Whip butter-flavored Crisco (TM or whatever). Add powdered sugar to desired consistency. (Not buttery enough for you? Add butter flavored extract from any good cake supply store).

She would bring a sample of each to the first client meeting, and most clients picked the Crisco for flavor and texture.

Now- on to your challenge- the only unknown is whether Crisco contains either corn or soy oil in it's "Vegetable Oil" ingredients. I can't get an answer off their website, but I bet a quick call to the manufacturer would do the trick. I did find one reference that said it was made from cottonseed oil, but better safe than sorry.

Problem #2- Powdered sugar: Yep, most powdered sugar has cornstarch added to it. Two reasons why: Less clumping during storage, and better thickening power in recipes. But if you don't need shelf life or thickening power, why add cornstarch? So when you make it yourself, the recipe goes like this:

-Pulse plain old granulated sugar in a food processor until it is as fine as you need it. Sift to remove lumps.

(What!?! You don't own a food processor? Use a VERY clean coffee grinder. It makes smaller batches and takes longer, but it does the job.) ;-)

Just to add a tuck and an half twist to the last trick- If you do want the thickening power and shelf-life of cornstarch, add a small amount of finely ground rice powder (rice, coffee grinder- you get the idea) to your homemade powdered sugar. Not as good as cornstarch, but a strong second-string player.

{Bonus hint, no extra charge- use rice to clean your coffee grinder of all the flavors and oils between using it for coffee or using it for herbs, sugar, etc. Running a batch of white rice cleans it better than anything I have found.}

Now for the half-court shot- if Crisco is not an option, or you can't get an absolute no corn, no soy guarantee, do what your great-great grandma did:
LARD.

No kidding. This giant of the worldwide baking industry has a undeservedly tarnished reputation in North America. Did you know it's lower in saturated fat than butter? Can't have the pork either? Tallow is made from beef and is the same thing, but harder to find.

Great-great grandma would never have used something as expensive as butter just to frost a cake. Lard and powdered sugar with a little vanilla would have won her the county bake-off title three years in a row. I'd add a little butter flavoring, though, just to be fancy.

A few extra steps and some creativity and you'll soon discover that the frosting in a can is a travesty. Homemade rocks!!!

Sorry for the long post, but I just stumbled across your question while doing a web search. I registered for the site just to put my two cents in. Hope it helps.