amazing lemon frosting recipe

http://www.foodandwine.com/recipes/lemon-cake-with-lemon-frosting

FROSTING

1. 1/2 pound unsalted butter, softened
2. 1 large egg yolk (optional)
3. 1 pound confectioners' sugar
4. 1/4 cup plus 2 tablespoons heavy cream
5. 1 teaspoon pure vanilla extract
6. Pinch of salt
7. 1 teaspoon finely grated lemon zest
8. 1 1/2 tablespoons fresh lemon juice
9. In a standing mixer fitted with the whisk, beat the butter and egg yolk at medium-high speed until creamy. Beat in the sugar at low speed. Beat in the cream, vanilla and salt, then beat at medium-high speed until fluffy, 3 minutes longer. Beat in the lemon zest and juice.
10. Spread 1 cup of frosting over a cake layer and top with the second layer; frost the top and sides. Let stand 1 hour; serve.