Turn the mixer on low, and carefully–carefully, I say!–slowly dribble in the sugar syrup, avoiding the spinning beater. Once the sugar syrup is incorporated, bump the speed up to medium-high and continue beating until the mixture is completely cool, about 5 minutes.☞ TESTER TIP: If the egg mixture is even the least bit warm, it'll melt the butter, and you'll be staring at a puddle--of frosting and your tears.