In a stand mixer fit with the paddle attachment on medium-high speed, cream butter until pale and fluffy, about 3 minutes.
Reduce mixer speed to low. Add 4 cups powdered sugar, heavy cream, vanilla, and salt. Continue mixing 2 to 3 minutes longer. If the frosting seems too soft, add the remaining powdered sugar 2 tablespoons at a time.
Cover and refrigerate the frosting until ready for use.
Notes
Butter: Refrigerated butter softens in about 30 minutes at room temperature. To soften butter in the microwave, cut each stick of butter in half, unwrap, and place on a microwave-safe plate. Heat the butter at 10% power in 30-second increments until softened but not melted, pressing the butter to check softness after each increment.
Yield: This recipe makes about 7 cups of frosting, enough for a 9-inch layer cake (with frosting in-between the layers), at least 2 dozen cupcakes, or one sheet cake.
Storage: Store buttercream frosting covered in the refrigerated for up to 4 days. Always bring buttercream to room temperature before using it (it won't spread well when chilled).