Line a 13x9x2-inch baking pan with foil or parchment paper; set aside. In a large mixing bowl beat butter and shortening with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, granulated sugar, and baking soda; beat until combined, scraping sides of bowl occassionally. Beat in the eggs and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Stir in chocolate and, if desired, nuts. Press dough evenly into the prepared pan.
Bake in a 375 degree F oven for 22 to 25 minutes or until golden brown and center is set. Cool in pan on a wire rack 1 hour.
Preheat oven to 325 degrees F. Holding securely to foil lining, gently remove cookies from pan and place on a cutting board, leaving cookies on foil lining. Cut crosswise into 9x1/2-inch slices. Place slices, cut side down, about 1 inch apart on an ungreased cookie sheet. Bake for 6 to 8 minutes or until cut edges are crispy. Carefully transfer cookies to wire rack (cookies will be tender). Cool. Makes 18.
Bake cookies as directed; cool completely. Place in a freezer container or bag and freeze for up to 3 months. Before serving, thaw for 15 minutes.