Preheat your oven to 350 degrees Fahrenheit. Line 2 baking sheets with parchment paper or silicone baking mats.
In a large bowl, whisk together the dry ingredients: all-purpose flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Using a stand mixer with the paddle attachment, cream together the butter, cookie butter, and sugars on medium speed for about 2 minutes.
Add the egg, beat, then scrape the mixture. Add the egg yolk and vanilla extract. On low speed, add the dry ingredients until just incorporated. Mix in the white chocolate chips and crushed Biscoff cookies.
Shape the dough into 8 large balls (about 1/2 cup each) and place them on the prepared baking sheets. Bake for 10-12 minutes until the edges and tops are slightly golden. Ensure enough space between cookies as they will spread.
Right out of the oven, cut a Biscoff cookie in half and gently press it into the center of each cookie. Optionally, garnish the tops with more white chocolate chips.
Let the cookies cool on a wire rack for 15 minutes, then transfer them to cool completely.
Store these delightful cookies in an airtight container at room temperature for up to 3-4 days.