Preheat the oven to 350°F (180°C). Spray a 9x9 square dish with non-stick spray and line it with parchment paper so that there is overhang on both sides.
Add melted butter and light brown sugar to a mixing bowl and whisk together.
Add eggs, one at a time, stirring well after each egg is added, and then add the separated egg yolk and vanilla extract and stir in well.
Sift all purpose flour into a separate bowl and add cornstarch, baking powder and salt and mix together.
Add the dry ingredients in with the wet and stir in until combined. Don't overmix.
Add broken up biscoff cookies and white chocolate chunks (or chips) and fold into the batter.
Transfer the batter to your prepared 9x9 square baking dish and smooth down.
Add biscoff cookie butter to a microwave safe bowl and microwave in 15-second intervals bringing it out to stir every 15-seconds until melted.
Spoon the melted biscoff cookie butter on top of the blondies and use a toothpick to lightly swirl it around the top of the blondies.
Place 8 whole biscoff cookies (4 on each side) along the top of the blondies.
Bake for 30 minutes until the edges are golden.
Allow the blondies to cool in the pan and then drizzle melted biscoff cookie butter on top and add a sprinkle of broken up biscoff cookies.
Remove the blondies from the dish using the parchment paper overhang to easily lift them out.
Cut into 16 squares and serve.