Preheat oven to 350°F (175°C) and line an 8-inch (20cm) square tin with baking paper.
Melt the butter, then whisk in both sugars until smooth.
Add the eggs and vanilla, mixing well.
Sift in the flour and salt, and fold gently until just combined. Stir in the chocolate chips.
Pour the batter into the prepared tin. Warm the Biscoff spread slightly so it’s easier to drizzle, then spoon it over the brownie batter. Use a knife or skewer to swirl it through.
Sprinkle crushed Lotus biscuits on top.
Bake for 30–35 minutes, until just set in the middle.
Cool completely in the tin before slicing into squares.