Directions:

1. Combine the cocoa powder and chopped chocolate in a small bowl. Pour the boiling water on top and whisk until mixture is completely combined and smooth. Set aside and let cool to room temperature (about 4 hours at room temperature or 1 hour in the refrigerator, whisking every 15 minutes if refrigerating).
2. In the bowl of a stand mixer fitted with a paddle attachment or using an electric hand mixer, combine the cake flour, salt, baking soda, and brown sugar on low speed for about 10 seconds until mixed. With the mixer on low speed, add the butter and continue to mix until butter is completely mixed in, 30 to 40 seconds, and the mixture resembles dough. In a separate bowl, whisk together the crème fraiche, eggs, and yolks. With the mixer on low speed, pour the liquids into the butter-flour mixture. Turn the mixer up to medium-high and beat for 2 minutes, until batter is fluffy and light. Stop the mixer a few times and scrape down the sides and bottom of the bowl. Turn the mixer to low speed and slowly pour the cooled chocolate mixture into the bowl. Mix for about 10 seconds to combine. Scrape the sides and bottom of the bowl and then mix on medium speed for about 30 seconds until the chocolate is thoroughly mixed in.
3. Pour batter into prepared pan and bake for 1 hour and 20 to 1 hour and 30 minutes, until the center of the cake springs back when you poke it in the middle.
4. Remove the cake from oven and let it cool in pan on a cooling rack.
5. When the cake is completely cool, remove it from the pan and trim the top with a serrated knife to level it (it will have rounded a bit in the oven—these scraps make for great nibbling). Again using a serrated knife, split the cake into two even layers (see page 00). Place the bottom layer on a cake plate or pedestal (or use a cake turner if you have one) and spoon 1 scant cup of buttercream on top; use an offset spatula to spread the frosting evenly to the edges of the cake.
6. Carefully place a second layer of cake on top of that and spoon on about 1 scant cup of buttercream. Spread this to the edges of the cake and then place the third layer on top of that. Spoon about 1 cup of buttercream on the top of the cake and spread this along the top and down the sides of the cake, smoothing the buttercream as well as you can and covering the entire cake with a thin layer of frosting. This is called a crumb coat (see page 00), and it will keep any loose crumbs from migrating to the surface of the finished cake. Spoon another cup or so of frosting on the cake and spread it evenly across the top and sides again. This is the final finishing layer of frosting. Use remainder of frosting to pipe an edge along the top and/or the bottom edge of the cake as desired.
7. The cake keeps at room temperature for up to 2 days in an airtight container.