Fresh berries yield superb flavor and color to baking—choose ones that are fragrant, ripe and red. You may substitute frozen berries provided they are the whole, quick-frozen variety. Just thaw them and puree. If use you frozen berries with added sugar, then adjust the sugar accordingly in the frosting or it may be too sweet. Are you are unfamiliar with Swiss-meringue buttercreams? Be sure your equipment is grease-free (mixer bowl, whip and paddle) by wiping them with a paper towel dampened with vinegar. Then take it step-by-step and follow the tips if the process goes awry. The pasteurized egg whites in liquid form found in cartons at the grocery store are convenient and saves time.