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1
Adjust oven rack to middle position and preheat oven to 350°F. Line 10- by 15-inch jelly roll pan with parchment paper. Sift flour, cocoa, and baking powder into small bowl; set aside.
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2
Place chocolate and butter in a microwave-safe bowl and microwave in 15 second increments, stopping to stir with a rubber spatula until completely melted and smooth. Set aside to cool slightly. Alternatively, place butter and chocolate in a large bowl set over a saucepan of simmering water (do not allow bottom of bowl to touch water). Heat, stirring constantly with a rubber spatula, until melted and smooth. Allow to cool slightly.
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3
In bowl of standing mixer fitted with whip attachment, whip sugar with eggs and salt on medium-high speed until light and thick, about 5 minutes. Remove bowl from mixer and fold in chocolate mixture with rubber spatula. Fold in flour mixture and then water until just combined.
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4
Spread batter onto pan. Bake until just firm, 10 to 15 minutes. Let cake cool about 10 minutes. Run knife along inside pan edge to loosen cake.
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5
Place clean kitchen towel over cake. Starting from the short end, carefully roll cake up with towel (with parchment paper attached), Let cake cool until just cooled.
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6
In bowl of standing mixer fitted with whip attachment, whip cream with sugar and vanilla on medium high speed until medium peak. Gently unroll cake, removing towel but leaving very end rolled to ensure a good grip. Spread cream over exposed surface of cake. Carefully re-roll cake into log, removing parchment paper as you roll up. Wrap tightly with plastic wrap to keep cake in log shape as you prepare ganache.
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7
In medium heat-proof bowl, heat chocolate with cream, stirring frequently, in microwave or over saucepan of barely simmering water until chocolate is melted and mixture is smooth. Chill mixture until thickened slightly, about 10 minutes.
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8
Remove plastic wrap from cake and trim both ends of cake. Place cake on wire rack set over rimmed baking sheet. Spoon ganache over cake. Using off-set spatula, smooth ganache over sides and ends of cake. Chill until ganache sets, about 1 hour. Slice and serve.
