In a large bowl, cream shortening and sugar until light and fluffy. Beat in eggs and vanilla. Combine the flour, oats, cocoa, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk, beating well after each addition. Beat in walnuts.
Spread batter into a greased 15x10x1-in. baking pan. Bake at 350° for 18-24 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
For frosting, in a small saucepan, combine sugar and milk. Bring to a boil over medium heat, stirring constantly. Cook and stir for 30-60 seconds or until sugar is dissolved. Remove from the heat; stir in chocolate chips and butter until smooth. Immediately spread over bars. Let stand until frosting is set. Cut into squares.