From various ATK recipes including one we recently tested.
16 tablespoons unsalted butter, softened
4 cups (16 ounces) confectioner’s sugar
1/3 cup buttermilk powder
2 teaspoons vanilla extract
¼ teaspoon salt
12 ounces cold cream cheese, cut into 12 equal pieces
Using stand mixer fitted with paddle, beat butter, confectioner’s sugar, buttermilk powder, vanilla, and salt on low speed until smooth, about 2 minutes, scraping down bowl as needed. Increase speed to medium-low and add cream cheese, 1 piece at a time, and mix until smooth, about 2 minutes.
Pair this icing with ATK's/Cook's Illustrated's Dark Chocolate Cupcakes recipe.