Preheat your oven to 180°C (350°F). Line a baking sheet with parchment paper.
In a large bowl, whisk the egg yolks with 50g of sugar until pale and creamy.
In a separate bowl, whisk the egg whites until soft peaks form. Gradually add 50g of sugar and continue whisking until stiff peaks form.
Gently fold the flour and cocoa powder into the egg yolk mixture, then fold in the egg whites.
Spread the batter evenly on the prepared baking sheet and bake for 10-15 minutes until the sponge is set.
While the sponge is baking, prepare the chocolate ganache by heating 100ml of cream to a simmer, then pour over 200g of chopped dark chocolate. Stir until smooth.
Once the sponge is baked, turn it out onto a clean kitchen towel and roll it up loosely while it's still warm. Let it cool completely.
Unroll the sponge and spread with 150ml of whipped heavy cream. Reroll tightly without the towel.
Cover the rolled sponge with the chocolate ganache. Use a fork to create a bark-like texture.
Chill in the fridge for at least an hour before serving.