1 lb Unsalted butter
1 c  Sugar
1/2 ts Salt
1/4 c  Condensed milk
1/2 ts Vanilla extract
3 1/2 c  All-purpose flour
1/2 c  Cornstarch

COMBINE BUTTER, sugar and salt in an electric mixer fitted with a paddle,
if you have one, and beat at medium speed until creamy. Add condensed milk
and vanilla and beat until incorporated. Reduce speed to slow and slowly
add the flour and then the cornstarch. Beat until incorporated. Divide the
dough unto 4 balls, wrap in plastic wrap and chill in the refrigerator for
1 hour, or until firm. Lightly flour a work surface and roll out the dough
to a 1/4-inch thickness. Keep lifting the dough and, if it sticks to the
surface, sprinkle with a little more flour. Preheat oven to 300F. Using a 2
1/2-inch round cookie cutter, cut out circles of dough, or use a knife to
cut dough into 2-inch squares. Place on a non-stick or lightly buttered
cookie sheet, and replace in the refrigerator to chill, about 15 minutes.
Place rack in the middle of the oven. Place cookies in the oven for 15 to
20 minutes. The cookies should color only slightly. Cool completely.