For chocolate sponge, preheat oven to 220C. Whisk eggs and caster sugar in an electric mixer until pale and triple in volume (8-10 minutes). Sift over flour and cocoa powder and gently fold until combined, then fold through melted butter. Pour into a greased and baking paper-lined 30cm x 42cm baking tray, smooth top and bake until cake springs back when lightly pressed (4-5 minutes). Turn onto a wire rack to cool.
Melt chocolate in a heatproof bowl over a saucepan of simmering water and set aside. Whisk egg yolks and sugar in an electric mixer until pale and frothy (4-5 minutes). Stir through melted chocolate and butter until combined. Whisk eggwhites and a pinch of salt until firm peaks form, then fold through chocolate mixture and set aside.
Cut chocolate sponge to fit the base and sides of a 8cm x 22cm straight-sided loaf pan, line pan with sponge and fill with chocolate mixture. Trim remaining sponge to fit top, cover with plastic wrap and refrigerate for at least 6 hours or overnight.
Meanwhile, for Cognac raisins, combine sugar and 2 tbsp water in a small saucepan over high heat, stirring, until sugar dissolves, then simmer for 2 minutes, add raisins and Cognac and remove from heat. Set aside to cool (raisins will keep refrigerated for up to 2 months, the flavour will improve with age).
Remove cake from mould and thickly slice with a warmed knife. Serve with Cognac raisins spooned over.