125 g (4 oz) plain dark chocolate, broken into small pieces
butter, for greasing
4 large eggs, separated
125 g (4 oz) caster sugar
200 g (7 oz) mascarpone cheese
150 g (5 oz) crème fraîche
seeds scraped from 1 vanilla pod or 1 teaspoon vanilla bean paste
250 g (8 oz) mixed summer berries, thawed if frozen
icing sugar, for dusting
Melt the chocolate in a heatproof bowl over a saucepan of gently simmering water, ensuring the bowl does not touch the water, then leave to cool.
Meanwhile, grease a 23 x 33 (9 x 13 inch) Swiss roll tin and line with nonstick baking paper. In a large bowl, beat the egg yolks and sugar with a hand-held electric whisk until pale and thick. Whisk the egg whites in a clean bowl until stiff. Fold the cooled chocolate into the egg yolk mixture, then the egg whites.
Pour the mixture into the tin and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 12–14 minutes until slightly springy to the touch. Cover with a piece of baking paper, lay a large board on top and invert the sponge on to the board. Remove the baking paper, trim the edges and leave to cool for 5–6 minutes.
Meanwhile, beat together the mascarpone, crème fraîche and vanilla in a bowl until thickened. Spread over the roulade and scatter with half the berries. Using the baking paper, roll up the sponge lengthways, then dust with icing sugar. Decorate with the remaining berries.