Heat oven to 180C. Melt 75 grams chopped chocolate in a heatproof bowl over a pan of gently simmering water, stirring until smooth. Remove and cool.

Beat butter and two sugars in a bowl until smooth, about five minutes. Beat in eggs, melted chocolate, vanilla and salt. Sift in flour and bicarbonate of soda and fold through, then fold in remaining chopped chocolate and nuts. Refrigerate for 30 minutes.

Using an ice-cream scoop, place eight balls of mixture on a baking tray lined with baking paper, allowing room for spreading.

Bake for 15 minutes, then gently transfer, using baking paper, to a wire rack to cool. Bake remaining mixture in batches, or freeze for future use.

To make white chocolate mousse, melt white chocolate as described above, and cool. Using electric beaters, beat cream, egg white and liqueur until just firm, then lightly fold in white chocolate. Refrigerate for a few hours or overnight.

To serve, place spoonfuls of white chocolate mousse on top of four up-turned biscuits and top with remaining biscuits. Gently press down until mousse squishes out, then serve.