Prepare oven and baking sheet: Heat oven to 450 degrees, with rack in bottom half. Coat a 12 ½-by-17 ½-inch rimmed baking sheet with cooking spray. Line with parchment paper; coat with cooking spray. Dust with cocoa powder and tap out excess.

Combine dry ingredients: Whisk together cocoa powder, flour, and salt in a bowl.

Combine eggs and sugar: Bring about 2 inches of water to a simmer in a medium saucepan. In the heatproof bowl of an electric mixer set over (not in) simmering water, whisk eggs, yolks, and granulated sugar until sugar has dissolved and mixture is warm to the touch. Attach bowl to mixer fitted with whisk attachment; beat on medium-high speed for 2 minutes. Raise speed to high; beat until mixture is pale and thick, about 4 minutes more.

Add dry ingredients: Sift cocoa mixture over egg mixture; using a large rubber spatula, carefully fold. When almost incorporated, pour melted butter down side of bowl; gently fold to combine.

Run a knife around sides of cake. Invert onto prepared towel, and remove parchment.

While the sheet cake is still warm, gently roll, starting from one short side, into a log, incorporating the kitchen towel. Let the cake cool completely, rolled. (This will give the cake a "shape memory," so it will be easier to roll again with filling.)

Unroll cake.

Spread filling over cake leaving a ½-inch border all around. Carefully roll cake to enclose filling (without towel), starting at a short end. Refrigerate 30 minutes (or up to 3 hours). Pour the ganache on the chilled cake, letting it drip down the sides for a rich, glossy finish. Chill briefly to set before slicing and serving.

Place chocolate pieces in a medium bowl. Heat cream in a small sauce pan to almost boiling. The milk will be steaming hot. Pour the hot cream over the chocolate pieces. Allow to stand for one minute before whisking into a smooth and glossy chocolate sauce. Allow to rest in the fridge until thickened slightly, about 30 minutes.