Preheat your oven to 375° F.
Place butter (cut into chunks) and sugars into a stand mixer and blend well until butter and sugars are fully incorporated.
Add vanilla. Add eggs one at a time until blended well with the sugar-butter mixture.
Add the flour, cornstarch, baking soda, and salt. Mix on low until the two are just combined, being careful not to over mix.
Break up Biscoff cookies into pieces and add to mixture. I do this by placing a couple cookies in my hand at a time and just crunching them with my hand over the bowl of cookie dough, allowing pieces of various sizes. Some will be more like crumbs, and some will be actual small pieces - that's fine. The variety makes it great.
Add white chocolate chips. Pulse together gently until all mixed in. Again, being careful not to over mix.
Scoop out portions of dough that all weigh about 2 oz each (about 2 tablespoons). After they are all divided, press down gently on each one, making them thick, flat disks (see picture above). Half of these are going to be the bottom of your cookies, the other half are going to be the tops. (You can make these larger or smaller by the way. To make mini's do about 1 oz each, and to make giant ones like a bakery, do about 3 oz each.)
Time to add the Biscoff cookie butter spread. Spoon out about 1 tablespoon onto half of the cookie dough disks. Take the empty halves and place them on top of your bottom halves that now have cookie butter scooped on top like a cookie butter sandwich.
Carefully pinch each one together, pressing around the sides. Use the palms of your hands to gently help reshape into a ball as necessary.
Once you've done this with all of the dough, place 12 balls per cookie sheet. If you make them the size as instructed, you should get about 12 generous cookies, but it can range from about 8-20 depending on how big you make them.
Place your cookie sheet in the fridge to chill for about 10-15 minutes to prevent them from spreading too much.
Place cookies in the oven and bake for 9-10 minutes. Let cookies cool.
Place about 3/4 cup cookie butter spread in a microwave safe bowl. Microwave for about 15 seconds until thin. Place in a drip bottle or in a plastic bag and cut off the tip to drizzle the tops of your baked cookies with additional cookie butter.
Enjoy! Store airtight and enjoy fresh for up to 5 days. These also freeze great!