Preheat the oven to 350 degrees Fahrenheit . Line 2 baking sheets with parchment paper or Silpat/silicone baking mats. Set aside.
In a large mixing bowl, whisk together the dry ingredients: all-purpose flour, cornstarch, baking powder, baking soda, and salt. Set aside.
2 cups all-purpose flour, 2 teaspoons cornstarch, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, 1/2 teaspoon sea salt NOTE: if using unsalted butter
Using a stand mixer fitted with the paddle attachment, cream the butter, cookie butter, and sugars on medium speed until combined, about 2 minutes.
14 Tablespoons salted butter, 2/3 cup creamy cookie butter, 1/2 cup brown sugar, 1/2 cup white sugar
Add in the egg and beat. Stop and scrape the mixture. Then, add the egg yolk and vanilla extract. On the lowest speed of the blender, add in the dry ingredients, stopping as soon as the flour is incorporated into the dough. Mix in the white chocolate chips and crushed Biscoff cookies.
1 large egg + 1 large egg yolk, 2 teaspoons vanilla extract, 1 1/4 cups white chocolate chips, 10 Biscoff cookies
Shape the dough into approximately 8 large dough balls (about 1/2 cup each).
Place the cookies on the prepared baking sheets. Bake for about 10-12 minutes or until the edges and tops look slightly golden. Be sure to leave enough space between the cookies as they will spread!
As soon as the cookie comes out of the oven, cut a Biscoff cookie in half, and gently press it into the center of each cookie. Garnish the tops with more white chocolate chips when they come out of the oven (optional).
4 Biscoff cookies for garnish
Let the cookies cool for 15 minutes on a wire rack. Then, transfer them to a wire rack to continue cooling completely.
Store in an airtight container at room temperature for up to 3-4 days.