In a small bowl, whisk the flour with the salt. In a medium bowl, using an electric mixer, beat the butter with the sugar at medium speed until creamy, about 3 minutes. Beat in the egg and vanilla. Add the flour and beat at medium-low speed until a very soft dough forms. Divide the dough in half, flatten into disks and wrap in plastic. Refrigerate the dough until firm, about 30 minutes.