1. Preheat the oven to 350 F and line two baking sheets with parchment paper. In a large mixing bowl, sift together the flour, baking soda, cornstarch, ground cinnamon and salt. Set aside.
2. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, beat the unsalted butter with the two sugars on high, for about a minute, until light and fluffy. Add the egg and egg yolk and beat again for another minute, scraping down the sides of the bowl with a spatula as needed. Add in the vanilla extract and combine. Next, add in the cookie butter and mix on high until combined. 
3. With the mixer running on low, gently add in the dry ingredients until everything is just combined. Use a large wooden spoon to stir in the chopped up Biscoff cookies. Place the cookie dough in the fridge, covered, for 30 minutes to an hour.
4. Remove the cookie dough from the fridge, and using a large cookie scoop, place the cookie dough balls about 2 inches apart on the prepared sheets. Bake the cookies for 9-10 minutes, until the edges have browned slightly.
5. Remove the cookies from the oven, and allow them to rest on the pan for 10 minutes before removing them to a wire rack to cool completely.

Place cooled cookies in a sealed container to store. They’ll stay fresh at room temperature for 4-5 days, and even longer (but crispier) or in the freezer for up to 2 months.