Work cold butter with hands until soft and cream in the sugar. Mix flours and salt together and work into butter. Knead gently until soft (not oily) and forms a ball.
Roll dough to 3/8-inch thickness on a cookie sheet, mark pieces with a ruler to make 1" x 2" cookies and prick each cookie with a fork three times. Chill dough at least one hour (but up to 48 hours is still OK).
Heat oven to 375 degrees and reduce heat to 275 degrees as soon as you put the cookies in the oven.
Cut cookies apart and place on baking sheets. Bake in center of the oven at 275 degrees for 30 - 40 minutes.