Start with the filling by placing the chocolate into a medium sized bowl and the cream into the pan set over medium high heat.
Once the cream has come to a boil pour over the chocolate and stir together until smooth. Cover the bowl with cling film and refrigerate until needed (has to be cold before can be used).
Preheat the oven to 180°C (160°C fan) and grease and line with parchment a 27x29 rimmed baking tray. Lightly grease the parchment and dust with a little flour, tapping out any excess.
Place the eggs and sugar into a large bowl and using an electric mixer whisk until the eggs are thick and pale, when the whisk is lifted from the bowl it should hold a ribbon on the surface.
In two additions sift in the flour and cocoa powder gently folding together to combine. Take a large spoonful of the batter and mix into the melted butter, pour this mixture back into the main bowl and fold together.
Pour the mixture into the prepared pan and gently level out. Bake in the preheated oven for 15 minutes or until the cake has risen and is springy to the touch. Place a slightly damp tea towel onto the work surface and place a large piece of parchment paper on top.
Sprinkle the parchment with a little caster sugar. When the cake has come out of the oven carefully invert it onto the parchment, peeling the parchment off the back off the cake. Whilst still warm wrap the cake into a tight roll (keeping the parchment and towel inside the roll).
Allow to cool fully in the roll and when ready to assemble you can finish off the filling.
Remove the cream mixture from the fridge and whisk until it hold soft peaks. Unwrap the cake and using a palette knife spread the cream across the cake leaving a couple cm around the outside clear.
Roll the cake back up, trim off the edges and place onto a serving plate. To finish dust with either icing sugar or cocoa powder and top with a few chocolate shavings.