Preheat your oven to 350°F (175°C) and line a jelly roll pan with parchment paper.

In a large bowl, whisk together flour, cocoa powder, sugar, and eggs until smooth. Stir in melted butter and vanilla until combined.

Pour the batter into the prepared pan and bake for 12–15 minutes, or until set but soft.

Immediately invert the cake onto powdered sugar-dusted parchment paper and roll it up tightly while warm; let cool completely.

Whip heavy cream with powdered sugar until soft peaks form. Unroll the cooled cake gently and spread the filling evenly before rolling it back up again.

For ganache, heat heavy cream until simmering, then pour over chopped dark chocolate; stir until smooth. Pour over the rolled cake and slice for serving.