Step 1

For sour-cherry syrup, combine cherry syrup and sugar in a saucepan over high heat, stir to dissolve sugar, then bring to the boil and cook until syrup coats the spoon (10-15 minutes). Set aside to cool, then refrigerate until chilled (1 hour).

Step 2

For roulade sponge, preheat oven to 190C and butter and line a 20cm x 30cm Swiss roll pan with baking paper. Melt chocolate and 80ml water in a heatproof bowl over a saucepan of simmering water, stirring occasionally until smooth. Meanwhile, whisk yolks and sugar in an electric mixer until pale and creamy, and whisk eggwhites in a separate bowl until soft peaks form. Fold chocolate mixture into yolk mixture, then fold in eggwhite in batches. Spread batter gently in prepared baking tray and bake until a skewer withdraws clean (25-30 minutes). Remove from oven, cover with a slightly damp tea towel for 5 minutes, then refrigerate in tray placed on a wire rack until cool (20-30 minutes).

Step 3

Dust cocoa over a large sheet of baking paper placed on 2 overlapping larger pieces of plastic wrap. Invert cake onto cocoa powder, remove tray and peel off baking paper. Spoon three-quarters of the cherries over sponge and combine remaining with some of the cherry syrup to serve on the side (any remaining syrup can be reserved for another use, such as drizzling over fruit salads and ice-cream).

Step 4

Whisk cream, crème fraîche and icing sugar in a bowl to firm peaks, then spread it over the cherries and sponge, leaving a 1cm border. Rolling away from you, and using the plastic wrap to lift and roll over, wrap the roulade until into a log shape (you may need someone to help you hold it in place as you roll; the sponge is delicate andwill crack a little but firms up in the fridge), then gently twist the ends of the plastic wrap to secure. Refrigerate roulade on a plastic tray until well chilled and firm (1-2 hours). Dust generously with cocoa, cut into thick slices and serve with extra cherries in syrup. This roulade is best served on the day it's made.