Melt the chocolate: Gently melt the dark chocolate using a double boiler or microwave in short bursts.

Separate the eggs: Place yolks in one bowl and egg whites in another, ensuring no yolk touches the whites.

Whisk the yolks: Beat yolks with caster sugar until pale, thick, and creamy.

Stir in cocoa and chocolate: Fold the melted chocolate and cocoa powder into the egg yolk mixture.

Whip the egg whites: Beat whites until stiff peaks form to help the roulade rise.

Fold whites into chocolate mix: Add a third of the whites first to loosen, then gently fold in the rest.

Bake the sponge: Spread onto a lined Swiss roll tin and bake at 180°C for 20 minutes.

Cool and prepare: Allow the sponge to cool under a sheet of parchment to keep moisture.

Spread with cream: Whip the double cream with vanilla and spread evenly over the sponge.

Roll carefully: Use the parchment to gently roll the sponge into a tight roulade.