Preheat the oven to 350°F (175°C). Line the bottom and sides of an 8-x-8-inch (20-x-20-cm) square baking pan with aluminum foil or parchment paper, leaving enough overhang on all sides of the baking pan to allow for easy removal at the end. Set aside.
In a medium bowl, stir together the flour and cornstarch. Set aside.
Using an electric hand mixer or a stand mixer, beat the butter, granulated sugar, and brown sugar on medium speed until smooth and creamy, 3 to 4 minutes. Scrape down the sides and bottom of the bowl as needed. Add the milk, vanilla, and salt; mix until incorporated for 1 minute.
Reduce the speed to low and gradually beat in flour mixture; mix until combined. Be careful not to overmix; the batter will be thick. Stir in the white chocolate chips and crushed Biscoff biscuits mix until evenly distributed. NOTE: you can reserve some crushed biscuits to add on top before baking, if desired.
Spread the dough in an even layer at the bottom of the prepared pan.
Place the Biscoff spread in a microwave save bowl and heat for about 15 seconds, then add small blobs of Biscoff spread on top of the blondie batter. Use a toothpick or butter knife to gently swirl the spread into the rest of the batter. If desired, top with more crushed biscuits.
Bake for 28 – 30 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs attached. It's super important NOT TO OVER-BAKE these blondies. The blondies may appear very soft, but they will set up as they cool.
Transfer the pan to a wire rack and cool completely, about 4 hours, before cutting.
Lift the foil out of the pan using the overhang on the sides and cut it into bars. Store at room temperature for up to 5 days.