Grainy Icing

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.

Carol H.
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Lori T. December 2, 2018
If your mother-in-law is actually living in Germany, using typical German baking products, the problem may lie in the powdered sugar. German powdered sugar is just that, very finely ground sugar. It doesn't contain cornstarch. Some companies will add a very tiny amount, to help avoid clumping, but most don't. If this is the trouble, then what your recipe will need is the addition of a bit of water to help hydrate the starch and dissolve the grains. Or you can use your food processor to reduce granulated sugar to a powdery form and use that instead. That's what I do.
PieceOfLayerCake December 2, 2018
If you're whisking all of the lumps out of the powdered sugar, my only instinct is that you may be over whipping the egg whites. When we over whip egg whites, you over coagulate the proteins and they bunch up into little granules. Whenever I'm whipping egg whites on their own (without sugar), I never go "stiff", only "firm". Its a subtle distinction, but the egg whites should still be smooth and glossy, when they start to look like cumulus clouds, it may be too stiff. I'd try whipping the whites less and see how that works.
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