Preheat the oven to 350°F. Spray a 9x13 inch baking pan with nonstick spray.
In a medium bowl, whisk together the flour, pie spice, baking soda, and salt; set aside.
Using an electric mixer, cream the butter and sugar on medium-high speed until smooth; beat in the egg and vanilla until combined. Beat in the pumpkin purée (the mixture may appear curdled). Reduce the speed to low, and mix in the dry ingredients until just combined. Stir in the chocolate chips.
Spread the batter evenly in the prepared pan. Bake until the edges begin to pull away from the sides of the pan and a toothpick inserted into the center comes out with just a few moist crumbs attached, 35 to 40 minutes. Cool completely in the pan.
Use a serrated knife to cut 24 squares.
Notes
If you can't find pumpkin pie spice, substitute 1½ teaspoons cinnamon, ¾ teaspoon ginger, ½ teaspoon nutmeg, and ½ teaspoon each allspice and cloves.
Make sure the cake has cooled completely before cutting. Sometimes it's easier to get nice, clean cuts if it is chilled in the refrigerator for a while first.