• 3 free-range eggs, separated
• 100 g (3½ oz) light muscovado or golden caster sugar
• 25 g (1 oz) cocoa powder
• 75 g (3 oz) self-raising flour
• 1 tablespoon strong coffee, rum or Tia Maria

Filling

• 200 g (7 oz) chocolate
• 1 tablespoon strong coffee, rum or Tia Maria
• 50 g (2 oz) unsalted butter, softened
• 2 free-range eggs, separated
• 25 g (1 oz) golden caster sugar
• pinch of salt

Preheat the oven to 180°C/350°F/gas mark 4. Line a Swiss roll tin, approx. 25 × 35 cm (10×14 inches), with buttered greaseproof paper. Put the egg yolks and three-quarters of the sugar into a pudding basin, set over a saucepan of barely simmering water. Whisk together until pale, foamy and much increased in volume.

Sift the cocoa powder and flour together, and fold gently into the egg and sugar mixture. Stir in the coffee or liqueur.

Whisk the egg whites with the remaining sugar and fold into the cake mix. Pour into the Swiss roll tin, level it off, and bake in the top half of the oven for about 10 minutes. Remove, and turn out onto a wire rack. Peel off the paper, and leave to cool.

To prepare the filling, put the chocolate and liqueur in a clean pudding basin over hot water, and melt the chocolate. Remove from the heat, cool slightly, and mix in the softened butter. Whisk the egg yolks and sugar until pale and ribbony. Whisk the egg whites with a pinch of salt until stiff, and fold both into the chocolate.

Line a 500 g (1 lb) loaf tin with clingfilm. Cut the sponge into pieces and use to line the tin, except for the short sides, leaving a piece of cake to fit over the top. Line the tin with the shiny surface of the cake towards the centre. Pour the filling in, and cover with the final slice of sponge.

Cover with clingfilm, and freeze for several hours until set. Turn out onto a board, slice, and serve on individual plates with a spoonful of crème fraiche or a dash of coffee liqueur, and a dusting of icing sugar.