Heat the oven to 375°F and line two baking sheets with parchment paper or silicon baking mats. Position the sheets so the dough bakes on a cool, lined surface rather than directly on hot metal.
Pulse 16 Biscoff cookies in a food processor or blender until finely ground, then set the crumbs aside; break 8 more cookies into rough chunks and reserve the final 8 for topping. The fine crumbs should resemble coarse flour, while the chunks should stay visibly uneven for texture.
Beat the cold cubed butter with the brown sugar and granulated sugar in a large bowl for 3 to 4 minutes. Stop when the mixture looks pale, thick, and matte rather than slick or melted.
Beat in the eggs and vanilla, scraping the bowl’s sides and bottom so no butter-sugar mixture is left behind. The mixture should become smoother and more cohesive after the eggs are fully incorporated.
Mix in the Biscoff cookie butter until it is evenly blended through the dough. The dough will take on a warm tan color and a strong caramelized spice aroma.
Add the flour, reserved fine cookie crumbs, cornstarch, baking soda, baking powder, and salt, then mix on medium-low only until the dry streaks disappear. Stop promptly once combined to keep the finished cookies soft and tender.
Fold in the broken Biscoff cookie pieces and white chocolate chips. The add-ins should look evenly scattered through a thick dough that holds its shape on the spoon.
Portion the dough with a large 1/4 cup cookie scoop and place the balls a few inches apart on the prepared sheets; gently roll them between your hands if desired. Even portions and open spacing help the cookies bake into similarly thick rounds.
Bake for 8 to 10 minutes until the edges are set and the centers remain slightly underbaked. Pull them when the tops show gentle cracks and the middle still looks pale and puffy.
Press half of a reserved Biscoff cookie and a few extra white chocolate chips into each hot cookie, then leave them on the baking sheets for 10 minutes before moving them to a wire rack. The centers finish setting on the sheet while staying soft and chewy.