http://cakecentral.com/recipes/7294/key-lime-cake-with-key-lime-buttercream

Key Lime Buttercream:
1 cup unsalted butter
1 cup Crisco
2 teaspoons real vanilla
1/4 teaspoon salt
8 cups powdered sugar
6 tablespoons key lime juice (more if desired for consistensey and flavor)
Directions for Key Lime Buttercream:
Beat butter and shortening together for about 5 minutes. Beat in vanilla and salt. Add powdered sugar 1 cup at a time alternating with the key lime juice.

That's the recipe I found on here but I was wondering if you could use all butter as I don't have any shortening. Does the acidity in the lime juice require shortening for some reason?

I don't see why it wouldn't work with all butter. Most of the time, people add shortening to a recipe to make it easier to pipe or reduce the "yellow" color, plus shortening holds up better in the heat. I prefer either all shortening (using hi ratio) or the half butter/half shortening as listed in the recipe.

I would mind the heat if you're going to use all butter, because if it's too hot your buttercream may go into melt-down!

My BC recipe is all butter, I never use shortening and it works fine. I just made it for my Dad's birthday.