I tried to make some buttercream frosting this morning with margarine sticks, soy milk, and powdered sugar (all at room temperature). it was going swimmingly until I added a little more powdered sugar, and then it seemed like the margarine began to seperate a bit from the frosting. it actually looked ok and tasted great, but the frosting ended up with a mottled type of coloring, like it was marbled. any idea what caused this? did I add too much powdered sugar and it just broke down the frosting on some level?