Line a 15x10x1-inch baking pan with foil, extending the foil over edges of pan. Grease foil. In the prepared pan arrange enough of the graham crackers, side by side, to completely cover pan bottom, breaking to fit as necessary.
In a small heavy saucepan cook and stir chocolate and shortening over medium-low heat until melted and smooth. Pour melted chocolate mixture over caramel layer in pan, spreading evenly. Sprinkle with nuts. Let stand until set. Using the edges of the foil, lift uncut candy out of pan. Cut into squares.