Grease and line a 30 x 20 cm (12 x 8 inch) Swiss roll tin. Whisk together the eggs and sugar until the mixer leaves a trail over the surface. Add the chocolate extract, sift in the flour and cocoa and fold in carefully.
Pour the mixture into the prepared tin. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes.
Place a clean tea towel on the work surface and put a piece of nonstick baking paper on top. When the sponge is cooked, turn it out on to the baking paper, roll it up carefully and leave to cool.
Mix together the crème fraîche, yogurt, icing sugar and chocolate sauce.
Unroll the roulade and spread the crème fraîche mix over it. Spoon the berries over the crème fraîche and roll up the roulade again. Dust with cocoa and serve immediately, decorated with extra berries.