Pastry chef Paola Velez makes her Italian Buttercream by starting with a thick, glossy meringue made with egg whites and sugar syrup. She then gradually incorporates softened butter, beating it until everything turns into a light and fluffy frosting. To make sure her meringue is light and airy, Velez wipes down the inside of the mixer bowl, the whisk attachment, and saucepan with a paper towel that has a splash of white vinegar on it to be sure they're extra clean. (Even a little bit of oil or butter in the bowl will prevent the egg whites from whipping up into the pillowy peaks needed to make this creamy buttercream.) The citric acid adds a touch of brightness, helping to balance the richness of the frosting.