Melt the Chocolate and Butter: Melt 1/2 cup of butter and 8 ounces of chocolate in a heatproof bowl over a double boiler or in the microwave. Set aside to cool slightly.

Whisk the Eggs and Sugar: In a large mixing bowl, combine 2 large eggs, 1 cup granulated sugar, and 1 teaspoon vanilla extract. Whisk until pale and fluffy.

Combine Chocolate Mixture: Pour the melted chocolate and butter mixture into the egg-sugar blend and stir until just combined.

Fold in Dry Ingredients: Gradually fold in 1 cup all-purpose flour, 1/3 cup cocoa powder, and 1/2 teaspoon salt until no streaks remain.

Stir in the Biscoff Cookies: Gently fold in 1/2 cup of crushed Biscoff cookies into the brownie batter.

Prepare the Baking Pan: Line an 8-inch square baking pan with parchment paper. Pour the brownie batter into the pan, spreading it evenly.

Swirl in Cookie Butter: Dollop spoonfuls of Biscoff cookie butter over the brownie batter and swirl it in with a knife.

Bake to Perfection: Preheat your oven to 350°F (180°C) and bake the brownies for 30-35 minutes.

Cool and Slice: Allow the brownies to cool in the pan for about 15 minutes, then chill in the fridge for 1 hour before slicing.