Several hours before baking:
Combine the flour and salt in a medium bowl. With an electric mixer, beat the butter and sugar until light and fluffy. Beat in the egg and vanilla. Beat in the flour mixture just until combined. Remove the dough from the bowl, gather it into two disks, wrap each in plastic, and chill until firm, 2 to 3 hours.
Heat the oven to 325°F. Cover your cookie sheets with parchment. Flour a work surface and have a little extra flour on hand for the rolling pin. Roll one dough disk 3/16-inch thick. Cut out the shapes you want (decorate with sprinkles before baking, if using) and transfer to the sheets, arranging them about 3/4-inch apart. Reroll the scraps to make more cookies. Bake until the edges turn golden, 13 to 16 minutes, depending on size. Let cool before transferring or decorating with icing. Repeat with the second disk of dough.