Preheat your oven to 170C/340F. Line an 8" square tin with baking paper and set aside.
In a large bowl, place melted butter and sugars and whisk together. Add in eggs and vanilla, and mix again to combine. Sift in the cocoa and flour, and mix completely. Tip in your biscuits and stir into the mixture.
Tip your brownie batter into your prepared tin, and bake for 25-30 minutes until the top is crisp and the middle still feels a little squishy under the crust. Remove from the oven and cool completely in the tin before you add the toppings.
To make the topping, melt both your Biscoff and white chocolate (in separate bowls) in the microwave in short 10-20 second bursts, stirring in between until completely melted and pourable. Pour the Biscoff over your brownies (still in the tin) and spread over the whole top. Drizzle white chocolate over the top, then use a skewer to drag the drizzle back and forth over the brownie to create a feathered pattern. Pop in the fridge to set for an hour or so.
To cut the brownies, run your knife under hot water for a few seconds, then use to slice your brownies. Clean your knife and re-dip in hot water in between each slice to get really clean cuts.
