In a small saucepan, cook flour and milk over medium heat, whisking constantly, until thickened and pudding-like and an instant-read thermometer registers 170℉ (77℃) 4 to 5 minutes. Remove from heat, and whisk in vanilla and salt. Pour into a small bowl; cover with a piece of plastic wrap, pressing wrap directly onto surface to prevent a skin from forming. Refrigerate until completely cool, about 1 hour.