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For the Cake: Preheat oven to 375u00b0 F and line a 9-inch by 13-inch rimmed baking sheet with parchment paper. Grease the paper. In the bowl of a standing mixer, beat the yolks with half of the sugar on high until the mixture is pale and very fluffy, about 5 minutes. Beat in the salt and vanilla, then the cocoa powder. Wash and dry the bowl and whisk, then beat the egg whites on medium high until soft peaks form. With the mixer running, gradually add the rest of the sugar and beat until stiff peaks form. Stir 1/4 of the egg white mixture into the egg yolks to lighten everything up. Then gently fold in the remaining egg whites. Transfer the mixture to the prepared baking sheet and bake until the cake springs back and is dry to the touch, 10 to 15 minutes. Let the cake cool in the pan for 5 minutes (the cake will deflate a bit). Then turn it out onto a clean kitchen cloth that has been dusted with cocoa powder. Gently roll the cake, towel and all, into a long cylinder. Let the cake cool completely in the towel while you prepare the ganache and filling. To Prepare the Ganache: Add the chocolate, butter, and salt to a bowl. Heat the heavy cream to a bare simmer, then add it to the chocolate. Stir until smooth and let sit at room temperature until it cools to a spreadable consistency. To Prepare the Filling: Heat the cream to a bare simmer and turn off the heat. Add the sugar, salt, vanilla bean, and instant coffee. Stir and let steep for 20 minutes. Remove the vanilla bean pod, making sure to scrape out all of the seeds into the milk, then add it to the room temperature mascarpone and stir to combine. To Assemble the Cake: Gently unroll the cake/towel. Spread the mascarpone mixture over the top and use the towel to help roll the cake into a tight cylinder. Cut a 2- to 3-inch piece off of one end at a diagonal to use as a stump. Gently move the cake to a parchment-lined baking sheet. Use the cooled ganache to attach the stump to the log and slather the ganache al over cake
