Preheat your oven to 350°F (180°C) and line an 8-inch square pan with parchment paper.

In a heatproof bowl, melt the butter and chocolate together until smooth.

In a large mixing bowl, whisk together the eggs, brown sugar, granulated sugar, and vanilla extract until pale and fluffy.

Incorporate the melted chocolate mixture into the egg mixture, stirring gently until combined.

Fold in the all-purpose flour, cocoa powder, and salt until just combined.

Stir in the crushed Biscoff cookies until evenly distributed.

Pour the batter into the prepared pan, smoothing the top.

Dollop Biscoff cookie butter over the batter and swirl it in for a marbled effect.

Bake in the preheated oven for 30-35 minutes, checking for doneness.

Remove from the oven and cool in the pan before chilling in the refrigerator for at least 1 hour.