Whisk together the flour, baking soda, cornstarch, and salt in a medium-sized bowl. Whisk in the pumpkin spices, cinnamon, nutmeg, ginger and cloves. Set aside.
Mix the butter with both sugars in a large bowl until creamy, light and fluffy.
Add egg and mix to combine. Add pumpkin puree and mix to incorporate.
Incorporate dry ingredients into the butter mixture and stir until the dough comes together.
Add the toasted walnuts and semisweet chocolate chips. Cover and refrigerate the dough for at least 2 hours.
Preheat the oven to 350F (180C). Line two large baking sheets with parchment paper.
Use an ice cream scoop to take equal portions of dough and place them on the prepared baking sheet. Flatten slightly. Repeat with remaining dough, keeping 2 inches (5cm) space between cookies.
Bake for 8-10 minutes or until slightly golden around the edges. Cool on baking sheets for 5-10 minutes.
While still warm, add chocolate chips and toasted walnuts on top of the cookies.
Remove to a cooling rack to cool completely.