Preheat oven to 350°F and line 2 large baking sheets with parchment paper and set aside.
In a large bowl or stand mixer (I highly recommend a stand mixer for this recipe) fitted with a paddle attachment, cream together 1 cup unsalted butter, 1 cup dark brown sugar, and 1 cup granulated sugar on medium speed (speed 4 on a stand mixer) for 2 minutes, scraping down the sides of the bowl halfway through.
Measure and spread the ¾ cup 100% pumpkin out on paper towels. I usually start with four of the select-a-size sheets on the bottom or two regular folded over, then press the pumpkin with another set of paper towels on top. Replace the top paper towel with two select-a-size sheets and flip the pumpkin so the new top towels are now on the bottom of the pumpkin. Discard the original bottom towels and replace them with two more, pressing to remove the excess moisture. You should have about 100g of pumpkin after squeezing.
Add the pumpkin to the mixing bowl, it should peel off the towel easily, but you can use a rubber spatula to assist. Mix until fully incorporated.
Add the 2 large eggs one at a time, mixing after each one just until the yolk is streaky throughout the dough. Add 1½ teaspoons vanilla extract and mix just until combined.
In a medium mixing bowl, whisk together the 4 cups all-purpose flour, 1 (3.4oz.) box instant pumpkin pudding mix, 2 tablespoons powdered milk, 1 tablespoon pumpkin pie spice, 1 teaspoon salt, 1 teaspoon ground cinnamon, and ½ teaspoon baking soda. With the mixing beating at low (stir) speed, slowly add the flour mixture a cup at a time. If you have a flour shield for your mixer, I recommend using it as this recipe creates a lot of dough. Increase to medium speed and mix just until combined, then remove from the mixer and use a rubber spatula to mix up any loose flour from the bottom of the bowl into the dough.
Add in all but ⅓ cup of the 11 ounces Guittard White Chocolate Chips and mix at low speed just until combined; be careful not to overmix.
Use a large #16 cookie scoop to portion out the dough into 2.5 ounce portions. Roll the balls of dough between your palms to round, then place on the prepared baking sheets, leaving 2 to 3 inches between each (8 cookies will fit in offset pairs on a 12x17-inch cookie sheet).
Use the palm of your hand to gently press the cookies down so they are about ¾-inch thick.
Bake the cookies one cookie sheet at a time for 5 minutes, pull out and press some of the reserved white chocolate chips into the tops of the dough, then return to baking for an additional 7 minutes or until the edges are just turning light brown. This is an optional step that helps prevent the chips from brownie but still lets them melt into and bind with the dough while baking. You may add them at the beginning or right out of the oven if that is your preference.
Remove from the oven and allow them to cool for 5 minutes on the pan before transferring them to a wire rack to cool completely.