Preheat oven to 350°F (175°C). Line a 10x15” jelly roll pan with parchment and lightly grease.
Beat eggs, sugar, and vanilla with a mixer for 5 minutes until thick and pale.
Sift flour, cocoa, baking powder, and salt into the egg mixture. Fold gently to keep volume.
Pour into prepared pan and spread evenly. Bake 10-12 minutes, until sponge springs back when touched.
Lay a clean kitchen towel flat and dust with powdered sugar. Invert hot cake onto towel and peel off parchment.
Starting from a short end, roll cake and towel together gently. Cool completely.
Whip cream, sugar, and vanilla to stiff peaks. Unroll cake, spread filling evenly, then re-roll (without towel).
For ganache: heat cream, pour over chopped chocolate. Stir smooth and spread over cake. Chill until set.