1. Preheat the oven to 350°F (175°C). Line a 9×9 inch baking pan with parchment paper, leaving an overhang on two sides, and lightly grease the parchment and sides of the pan.

2. In a microwave safe bowl or small saucepan, melt the unsalted butter until just liquefied. Add 0.5 cup of the Biscoff spread and whisk until smooth and combined, then set aside to cool slightly.

3. In a large mixing bowl, whisk together the light brown sugar and granulated sugar until no lumps remain. Pour in the melted butter and Biscoff mixture and whisk until thick and glossy.

4. Add the eggs and egg yolk one at a time, whisking well after each addition, then whisk in the vanilla extract until the mixture is smooth and slightly thickened.

5. In a separate bowl, whisk together the all purpose flour, baking powder, and fine sea salt. Add the dry ingredients to the wet ingredients and gently fold with a spatula just until no visible streaks of flour remain. Do not overmix.

6. Fold the chopped Biscoff cookies into the batter, reserving a small handful for sprinkling on top. Spread the batter evenly into the prepared pan, smoothing the surface with an offset spatula.

7. Warm the remaining 0.5 cup Biscoff spread in the microwave for 10 to 15 seconds until just pourable. Drizzle it over the top of the batter in ribbons, then use a knife or skewer to gently swirl it into the surface. Sprinkle the reserved cookie pieces over the top.

8. Bake for 25 to 30 minutes, or until the edges are set and golden and the center looks just barely set with a slight jiggle. A toothpick inserted near the center should come out with a few moist crumbs, not wet batter.

9. Place the pan on a wire rack and let the blondies cool completely in the pan for at least 1 hour. Use the parchment overhang to lift the slab from the pan, then slice into 9 large or 16 smaller squares and serve.