Begin by preheating your oven to 350°F (180°C). In a heatproof bowl, combine butter and semi-sweet or dark chocolate. Melt them together using a double boiler or microwave in 30-second intervals until smooth and glossy.

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

Slowly pour the melted chocolate mixture into the egg and sugar blend, stirring gently to combine.

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

Incorporate the crushed Biscoff cookies into the brownie batter, folding gently.

Line an 8-inch square baking pan with parchment paper and pour the brownie batter in, smoothing the top evenly.

Dollop spoonfuls of creamy Biscoff cookie butter over the top and swirl it into the batter with a knife.

Bake the brownies for 30-35 minutes, checking for a set top and edges pulling away from the sides.

Let your brownies cool completely in the pan on a wire rack, then transfer to the fridge and chill for at least an hour.