Preheat the oven to 180°C, 160°C fan. Line a 20 cm square tin with baking parchment. In a large bowl whisk the egg yolks with half the sugar until pale and thick. This builds structure and richness before moving to the meringue stage.

In a clean bowl whisk the egg whites until soft peaks form, then gradually add the remaining sugar and beat to a glossy meringue. Fold gently into the yolk mixture, keeping as much air as possible for a light sponge.

Sift the flour over the mixture and fold carefully. Divide the batter into two bowls. Stir ground walnuts into one half and cocoa powder into the other. Spoon in alternating sections into the tin and level lightly before baking.

Bake for 20 to 25 minutes until springy to the touch. Cool briefly in the tin, then transfer to a rack. Once cold, cut into small cubes. Set aside while preparing the syrup.

In a small saucepan combine water, sugar and lemon zest. Bring to a gentle boil until the sugar dissolves. Remove from heat, stir in rum and allow to cool. The syrup should be fragrant but not overpowering.

Whisk egg yolks, sugar and cornflour in a bowl. Heat the milk until just steaming, then gradually pour over the yolk mixture while whisking. Return to the pan and cook gently, stirring constantly, until thick and smooth. Stir in vanilla and cool with cling film touching the surface.

Mix raisins with rum and leave to plump for at least 15 minutes. Chop the walnuts finely so they distribute evenly through the layers. Move on to assembling.

In a glass serving dish scatter a layer of sponge cubes. Brush generously with rum syrup. Spoon over custard, then sprinkle walnuts and raisins. Repeat layers until ingredients are used, finishing with custard. Chill for at least 1 hour to allow flavours to settle.

Heat the cream with sugar until just simmering. Pour over chopped dark chocolate and stir until glossy. Allow to cool slightly so it thickens but remains pourable.

Spoon generous portions into bowls, drizzle with warm chocolate sauce and crown with whipped cream. Traditionally it is scooped rather than sliced, celebrating its rustic elegance. Serve slightly chilled for the fullest flavour.