Melt the chocolate and butter in a heatproof bowl over simmering water for 5–7 minutes, or in the microwave.

Whisk together eggs, sugar, and vanilla extract until pale and fluffy, approximately 3–5 minutes.

Combine the melted chocolate mixture with the egg mixture until fully incorporated.

Sift flour, cocoa powder, and salt into the batter and gently fold until no flour streaks remain.

Stir in crushed Biscoff cookies until evenly distributed throughout the batter.

Line an 8-inch square baking pan with parchment paper and spread the brownie batter evenly.

Dollop Biscoff cookie butter on top of the batter and swirl it using a knife.

Preheat oven to 350°F (180°C) and bake for 30–35 minutes, checking doneness with a toothpick.

Cool the brownies in the pan for 30 minutes, then chill in the refrigerator for clean cuts.