Preheat the oven to 190°C / 375°F / Mark 5. Line a 10 × 14 in / 25.5 × 35.5 cm Swiss roll tin with buttered greaseproof paper. Put the eggs and 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. 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 on to a wire rack. Peel off the paper and leave to cool. To prepare the mousse melt the chocolate in a clean pudding basin over hot water, and beat in the coffee or liqueur until thoroughly mixed. Remove from the heat, cool slightly, and mix in the softened butter. Whisk the egg whites with a pinch of salt until stiff and fold into the chocolate.
Cut the sponge into pieces and use to line a 1 lb / 455 g loaf tin, leaving a slice 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 cling film or foil and refrigerate for several hours until set. Turn out on to a board, slice and serve on individual plates with a spoonful of crème fraîche and a dusting of icing sugar.