Heat the oven to 180C/160C fan/gas 4. Butter and line an 18-20cm square brownie tin. Put the butter and dark chocolate in a large, heatproof bowl set over, but not touching, a pan of barely simmering water and gently melt until smooth and glossy. Alternatively, melt in short bursts in the microwave. Set aside to cool slightly.

Put the eggs and sugar into a large bowl and use electric beaters to whip for 5-6 mins until pale, mousse-like and doubled in volume. When you lift the beaters, it should leave a trail on top of the mixture. With the mixer running on low, pour in the cooled chocolate mix, then whisk until combined.

Scatter over the flour, cocoa powder and a pinch of salt, then use a spatula to fold the mixture together until thick and chocolatey. Crumble in most of the biscuits, keeping them in medium-sized pieces, reserving a few whole ones for the top. Fold gently in, then scrape the batter into the tin.

Spoon the Biscoff spread on top of the brownies, using a skewer or knife to swirl most of it into the batter, leaving a few blobs for pockets of Biscoff in the finished brownie. Press the reserved biscuits gently into the top. Bake for 35-40 mins or until the brownie is set around the edges and has a shiny and lightly cracked top, with a gentle wobble in the centre of the tin.

Leave to cool completely in the tin, or even better, leave in the fridge overnight before slicing into squares to serve. Will keep for five days in an airtight container.