Heat the oven to 350°F/175°C, and line an 8-inch/20 cm square baking pan with parchment paper, leaving an overhang on two sides for easy removal.

In a large heatproof bowl, combine the butter and dark chocolate. Microwave in 30-second intervals, stirring after each, until completely melted and smooth. Let cool slightly.

Whisk in the granulated sugar, brown sugar, and eggs until the mixture is glossy and well combined. Sift in the cocoa powder, flour, and salt. Stir with a rubber spatula just until no streaks remain. Fold in half of the chopped chocolate.

Pour half of the batter into the prepared pan and smooth the surface. Spoon 3 tablespoons of the Biscoff spread over the batter and swirl lightly with a toothpick or the tip of a knife. Spread the remaining batter on top and smooth again. Transfer the remaining 3 tablespoons of Biscoff spread to a piping bag or small zip-top bag. Snip the tip and pipe diagonal stripes across the surface. Drag a toothpick through the lines in the opposite direction to create a marbled pattern.

Sprinkle the remaining chopped chocolate and a handful of broken Biscoff cookies over the top.

Bake for 25 to 30 minutes, or until the edges are set and the center is just barely jiggly. A toothpick inserted should come out with moist crumbs but no raw batter.

Let cool completely in the pan on a wire rack. Refrigerate for at least 3 hours, or freeze for 2 hours, until fully set.

Use the parchment overhang to lift the brownies from the pan. Slice into squares with a sharp knife, wiping between cuts for clean edges. Store in an airtight container at room temperature for up to 5 days, or refrigerate for up to a week.