Bake time: 1 hour plus overnight chilling.
Preheat the oven to 190°C (170°C fan)/375°F/Gas mark 5 and line a 20cm (8in) square baking tin with nonstick baking paper.
Put the melted butter and both sugars into a large mixing bowl and beat together with an electric hand mixer for about 2-3 minutes until fully combined and no lumps remain. Add the eggs and continue to beat until thick and fluffy (this could take from a few minutes up to 15 minutes, depending on the temperature of the ingredients and your equipment). Mix in the melted chocolate until just combined.
Sift in the flour, cocoa powder and salt, then gently fold in with a wooden spoon or rubber spatula until no streaks of flour remain. Fold in the white chocolate chips until evenly distributed.
Transfer half the batter into your prepared tin and smooth it out to the edges. Place the 16 biscuits in lines across the batter. There will be 1 biscuit per square.
Scoop the remaining brownie batter on top of the biscuits, smoothing it out gently. Sprinkle the broken biscuits evenly over the top of the batter.
Bake for 25-30 minutes until the edges are cracked and the middle no longer wobbles. Leave to cool completely in the tin, then chill in the refrigerator overnight for best results. Cut into 16 squares and enjoy!
Store in an airtight container at room temperature or in the refrigerator for up to 5 days.
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