1Heat oven to 300°F. In large bowl, beat butter, shortening and sugars with electric mixer on medium speed until creamy, or mix with spoon. Stir in flour and salt. (Dough will be dry and crumbly; use hands to mix completely.)
2Roll dough into 9x6-inch rectangle on lightly floured surface. Cut into 1 1/2-inch squares. Place about 1 inch apart on ungreased cookie sheet.
3Bake cookies about 25 minutes or until set. Remove from cookie sheet to wire rack.