Preheat oven to 350°F and line a 10x15 baking sheet with parchment paper. Set aside.
In the large bowl of a stand mixer, whip egg whites until soft peaks form. Set aside.
In a separate large bowl, whisk together egg yolks and sugar until well combined and a pale yellow color. Add vanilla and whisk to combine. Set aside.
In a medium bowl, whisk together cake flour, cocoa powder, baking powder, espresso powder, and salt.
Add the dry mixture to the bowl of egg yolk mixture and stir to combine - it will be very thick!
Add half of the egg whites to the mixture and stir vigorously to combine completely.
Gently fold in the other half of the egg whites. Take your time and, again, be gentle - you don't want to eliminate all the air you whipped into them.
Pour batter onto the prepared pan, gently using a spatula to evenly distribute the batter.
Bake for 10-12 minutes, or until the cake is spongey and bounces back when touched.
While it's baking, lay out a flour sack or kitchen towel on the counter and sprinkle it with a layer of powdered sugar.Tip: I like to use my flour sifter so that it's a relatively even layer and I don't miss any spots!
Let the cake cool for a minute on the baking sheet, then carefully turn it out onto the towel and remove the parchment paper from the bottom.
Starting from one of the short ends, roll the cake up (rolling the towel with it as you go). Lay the wrapped cake seam side down and let cool completely.