Preheat oven to 300 degrees F. In a large mixing bowl, cream butter and sugar until fluffy, with mixer on low speed, about 2 minutes. Beat in egg yolk. Add flour and salt and stir until well blended. Turn mixture out onto a floured pastry sheet and knead until smooth. Press dough into a 6 x 11-inch baking dish, until 3/4 inch thick, and prick all over with toothpick or fork. Bake, uncovered, in preheated oven for about 1 hour, or until lightly brown. Cut into rectangles or strips.