I like Serious Cakes BC recipe and I make some up last night and left it on the counter, it's been really warm here and in fact the A/C is struggling to keep the house under 75 degrees.

Normally I like my BC cold and would pop it in the frig but I figured since so many ppl keep it on the counter so could I, plus was gonna need it in 24 hrs to frost the cake. (frosted cake and all left over frosting is in the frig now)

when I got home the frosting had yellowish clear "gook" (kinda how sour cream will get that water that build on the top, or the oil in peanut butter)

Now I only had 1 stick of real butter, but I had margarine on hand so I use both butter and margarine. Do ya think the gook was from the corn syrup or from the margarine

below is the recipe

Serious Cakes' Buttercream Recipe

22 tbs salted butter, softened (about 3 sticks)
2/3 c. Crisco shortening
5-8 tbs heavy cream
2 tsp clear vanilla
2 lbs powdered sugar (about 8 cups)
1-3 tbs corn syrup

It was the cheap margerine. I believe it usually contains about 54% vegetable oil spread and the rest is water and so it will tend to separate ( just like the condensation that country crock margarine collects on top). I would stick with real butter.