I read a book a few years ago called The Education of Patience Goodspeed and one of the characters, Fanny Starbuck, makes this shortbread. I made a copy of the recipe and just found it.
After I baked it, it was the best shortbread I have ever tasted! Here is the recipe:
1 cup butter
1/4 teaspoon salt (only if unsalted butter is used; otherwise omit salt)
1/2 cup sugar
2 cups flour
Preheat oven to 325 degrees F. Cream butter and sugar. Mix in flour; knead gently until soft. Roll dough out 1/4-to 1/2-inch thick with floured rolling-pin and slice into rectangles or cut into shapes with cookie cutter.
Bake for 15-20 minutes until just barely brown, turning cookie sheet halfway through baking period for evenness.