Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
Step:1 Whisk eggs and sugar until thick and pale.
Step:2 Sift together cocoa powder and flour, then gently fold into egg mixture.
Step:3 Spread batter evenly on the prepared sheet and bake for 10-12 minutes.
Step:4 While baking, prepare the chocolate ganache with heavy cream and chocolate.
Step:5 Roll the warm sponge cake in a towel to prevent cracking.
Step:6 Unroll the cake and spread a layer of chocolate cream filling.
Step:7 Roll the cake back up and cover with chocolate ganache.
Chill until set and slice to serve with a dusting of cocoa powder.