In a bowl, cream shortening and sugar. Add eggs, one at a time, beating well after each addition. Combine flour, soda, salt and oats; stir into creamed mixture; set aside. For filling, combine chips, milk and butter in a saucepan. Melt over low heat, stirring until smooth. Cool slightly. Stir in walnuts and extract. Press two-thirds of oatmeal mixture into bottom of a greased 15x10x1-in. baking pan. Cover with filling and sprinkle remaining oatmeal mixture on top. Flatten slightly. Bake at 350° for 20 minutes.