1. Preheat the oven to 350F.
2. Combine the cake mix, 1 egg, butter and 2 teaspoons pumpkin-pie spice and stir until mixture is crumbly. Press onto bottom of a 15x10-inch nonstick baking pan.
3. Place the cream cheese in a large bowl. Beat with an electric mixer set at medium speed until fluffy. Add the sweetened condensed milk and beat until mixture is smooth. Add the remaining 2 eggs, remaining 2 teaspoons pumpkin-pie spice, the pumpkin puree and salt and beat well. Pour evenly over the cake mix layer. Sprinkle with the walnuts.
4. Bake until the center is set, 30 to 35 minutes. Cool in pan on a wire rack. Chill, covered, for 2 hours. Cut into bars. Refrigerate leftovers.
Variation
For a mousse-like dessert with a crumb topping, simply reverse the layers of this bar cookie. After pouring the pumpkin mixture into a well-greased baking pan, crumble the cake mix mixture on top, then bake as directed. Check to see if it is set in the center and if not, bake for up to 10 minutes longer. Let cool, chill, and spoon into dessert cups.