In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium-low speed until smooth. Reduce mixer speed to low; add confectioners’ sugar, about 1 cup (120 grams) at a time, alternately with cold cream, about 1 tablespoon (15 grams) at a time, beating until smooth after each addition. Increase mixer speed to medium, and beat until smooth and fluffy, 1 to 2 minutes. Beat in pie spice and vanilla.