To make the chocolate cake, preheat the oven to 425ºF. Place a piece of parchment paper or a silicone mat on a 17x12 sheet pan, and rub butter on it to grease.In a large bowl, whisk to combine the cocoa powder, flour, and salt.Bring an inch of water to a simmer in a medium saucepan. Combine the eggs and sugar in a heatproof glass bowl, then place over the simmering water (make sure the water doesn’t touch the bowl). Using a hand mixer, beat the eggs and sugar together over medium speed for about two minutes, until the eggs are thick, pale yellow, and warm to the touch (if you have a thermometer, you want the eggs to be about 120ºF. If you don’t have a thermometer, just touch the mixture with your finger every once in a while until it feels like hot tap water). Remove the egg bowl from the heat and continue beating at medium high speed for another 3 minutes, until it’s thick and airy, and has reached the ribbon stage. That means when you drag and drizzle a spoonful of the liquid, it shouldn’t settle back into the liquid for a good 5–10 seconds. Mix in the melted butter.Add the dry ingredients to the mixture, and gently fold it into the eggs with a spatula, working quickly. Spread the batter onto the buttered parchment or silicone mat, leaving an inch from the edges. Bake the cake for 5–6 minutes, until springy to the touch.Place a piece of parchment paper on a flat surface and sprinkle lightly with cocoa powder. Flip the cake onto the parchment, then remove the parchment or silicone mat that the cake baked on. Gently roll the cake up into a log while it’s still warm. This is like muscle memory for the cake, and it will roll easier again later.To make the filling, combine the heavy cream, sugar, and vanilla extract in a large bowl, and whip with a hand mixer to stiff peaks, about 5 minutes on medium speed. Unroll the cooled cake, then spread the cream all over, leave a 1-inch border around all edges. Roll the cake up gently (leaving the parchment paper behind), then place on a wire rack.To make the chocolate ganache, microwave the cream and chocolate in a heatproof bowl in 30-second intervals, stirring after each time, until combined. Add the tablespoon of milk, which will give a more pourable consistency. Pour the chocolate ganache evenly over the cake, then refrigerate for 20 minutes before serving, so the ganache can firm up.The cake is now ready to be sliced and served. Enjoy!