Preheat oven to 350°F Grease a 13 x 9 x 2-inch baking pan.
Combine cake mix, nuts and butter until well mixed. Press mixture into bottom of prepared baking pan.
In a large bowl, combine pumpkin, evaporated milk, sugar, eggs, pumpkin pie spice and salt. Beat with an electric mixer on medium speed for 2 minutes or until thoroughly combined. Spread over crumb mixture.
Bake for 50 to 55 minutes, or until set and tip of knife inserted into center of pan comes out clean. Cool on a wire rack.
Cut into squares and top with sweetened whipped cream if desired.