This is one of the most delicious chocolate cakes in the book but I found it a bit too tender so I have gone back to my original way of mixing layer cake batter (please see below). I also thought it would be of interest to know that the acidity of the brown sugar, crème fraîche, and unsweetened chocolate fully activates the baking soda leaving no characteristic slightly soapy taste.
I am bolding the headings and changes and suggest that you print out this page and insert it in the book. Mix the Chocolates and Water In a medium bowl, whisk the chocolate, cocoa, and boiling water until smooth. Cover with plastic wrap to prevent evaporation and cool to room temperature, about 1 hour. To speed cooling, place it in the refrigerator. Bring it to room temperature before proceeding. Mix the Liquid Ingredients In another bowl, whisk the eggs, yolks, crème fraîche, vanilla, and half the cooled chocolate mixture, just until lightly combined. Make The Batter In the bowl of a stand mixer fitted with the flat beater, mix the flour, sugar, baking soda, and salt on low speed for 30 seconds. Add the butter and the remaining chocolate mixture. Mix on low speed until the dry ingredients are moistened. Raise the speed to medium and beat for 30 seconds. Scrape down the sides of the bowl. Starting on medium-low speed, with the mixer off between additions, add the egg mixture in two parts. Beat on medium speed for 45 seconds after each addition to incorporate the ingredients and strengthen the structure.The batter will be slightly fluffy. Scrape down the sides of the bowl. Using a silicone spatula, scrape the batter into the prepared pans and smooth the surface evenly with a small off set spatula. Each will be almost half full (23 ounces/650 grams).