In a small bowl, sift the flour with the cocoa powder and salt. In a standing mixer fitted with the paddle attachment, beat the butter with the sugar at medium-high speed until it is well blended. Beat in the dry ingredients at low speed until just combined. Add the vanilla extract and beat just until a soft dough forms, about 5 seconds. Press the pastry into a disk, wrap in plastic and refrigerate until the dough is firm, about 1 hour.