Prepare classic vanilla buttercream first, before adding in chocolate components, as follows:
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3/4 cup unsalted butter, at room temperature
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1/4 cup white vegetable shortening
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1/2 cup milk (I used 2 percent)
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1/4 tsp. salt
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1 and 1/2 tsp. vanilla extract
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2 pounds confectioner's sugar, sifted (If you use Domino brand "10x" and you don't plan to use the frosting in a piping bag, you can probably easily get away with no sifting! Yay! Seems like every baker I know hates sifting powdered sugar.)