In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium-low speed until smooth. Gradually add confectioners’ sugar, about 1 cup (120 grams) at a time, alternately with crème fraîche, about 2 tablespoons (30 grams) at a time, beating until combined after each addition. Increase mixer speed to medium, and beat until smooth and fluffy, about 3 minutes.