Preheat the oven to 350ºF (177ºC) and line an 8X8" (20X20cm) baking dish with overlapping pieces of parchment paper.
Add the softened butter and brown sugar to a large mixing bowl and beat them until creamed together - approximately 1-2 minutes. An electric or stand mixer works best.
Add the egg, egg yolk, cookie butter, and vanilla extract to the bowl and beat again until well combined, scraping down the sides of the bowl as needed. Set aside.
Add the flour, baking powder, and white chocolate chips to a small bowl and stir well.
Add the dry ingredients to the blondie batter a little at a time, mixing after each addition. Stir just until combined - do not overmix!
Spread the cookie butter batter into the bottom of the prepared baking dish, smoothing it into an even layer.
Sprinkle the crushed Biscoff cookies on top, pressing them in very gently.
Bake for 30-35 minutes, or until a toothpick inserted into the middle comes out clean. They should be golden brown with crinkly edges.
Allow the blondies to cool at room temperature before slicing - about 1-2 hours.
Slice into squares, serve and enjoy!
Video
Notes
Storing: Biscoff blondies will keep in an airtight container at room temperature for 3-4 days, or in the fridge for up to 5 days. Blondies also freeze well - just wrap them individually in plastic wrap, place in a freezer bag, and freeze for up to 3 months.