Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides.

In a large microwave-safe bowl, melt the butter. Add brown sugar and granulated sugar and whisk vigorously for 2 minutes until combined and glossy.

Add the eggs, egg yolk, and vanilla extract. Whisk until the mixture is smooth and slightly thickened.

Whisk in 1/4 cup of the Biscoff spread (cookie butter) into the wet batter.

Add flour, cornstarch, and salt. Switch to a spatula and fold gently until just combined. Do not overmix.

Fold in the white chocolate chips and half of the chopped Biscoff biscuits.

Pour the batter into the prepared pan and smooth the top. Drop teaspoon-sized dollops of the remaining Biscoff spread over the top and swirl with a knife.

Press the remaining biscuit pieces into the top of the batter.

Bake for 25–30 minutes, or until the top is golden and set but the center still jiggles slightly. Do not overbake.

Allow to cool completely in the pan on a wire rack before lifting out and slicing.