Beat butter at medium speed with an electric mixer until creamy; gradually add 1 cup powdered sugar, beating at low speed until blended.
Add milk, beating until blended. Gradually add remaining powdered sugar, beating until blended. Stir in grated orange rind.
Lemon Buttercream Frosting: Substitute 2 tablespoons grated lemon rind for orange rind.
Lime Buttercream Frosting: Substitute 2 tablespoons grated lime rind for orange rind.
