My German mother-in-law gave me the recipe for the icing that goes on her Frankfurter Kranz. It consists of 250 g of unsalted butter, 150 g powdered sugar, 2 packages of vanilla sugar, 3 eggs. Two egg whites are whipped until stiff, while the two yolks and remaining egg are combined with the powdered sugar and vanilla sugar. The butter is creamed and all is combined. However instead of it being smooth (like my M-I-L) it looks grainy. I’ve played with the temperature of the butter, and the order I add the ingredients, it still comes out grainy/small lumps. Any help would be appreciated.