Line a 20 cm / 8-inch square baking pan with parchment paper, leaving an overhang on two sides for easy lifting later.

In a large mixing bowl, stir together the ground walnuts, crushed biscuits, sugar, vanilla extract, and salt until evenly combined.

Add the water a little at a time, mixing with your hands, until the mixture holds together when you squeeze a handful, like wet sand.

Divide the mixture in half. Press one half firmly and evenly into the bottom of the lined pan.

Mix the cocoa powder and remaining 2 tbsp water into the second half until you get a uniform dark brown dough.

Press the cocoa layer on top of the plain layer, smoothing the surface with the back of a spoon. Chill for 30 minutes.

Melt the chopped dark chocolate and oil together in a heatproof bowl set over a pan of barely simmering water, stirring until smooth and glossy, about 3 to 4 minutes.

Pour the melted chocolate over the chilled base and tilt the pan so it spreads into an even layer.

Refrigerate for at least 2 hours, until the glaze is set firm and the layers feel solid when pressed.

Lift the whole block out using the parchment overhang. Cut into 20 small squares with a warm, sharp knife, wiping the blade clean between cuts.