Cream butter and sugar together until well mixed. Add egg and vanilla. Add flour and salt.

Separate dough into two balls. Wrap each ball in waxed paper and chill for at least one hour, or up to a few days.

Preheat oven to 375 degrees.

Roll dough out to 1/8-inch thickness. Cut out with cookie cutters.

Bake for 7-10 minutes (cookies will brown just a tiny bit on the bottom and around the edges, but not on top).

Let cookies cool; spread with frosting, and decorate with sprinkles as desired while frosting is still moist.