• Preheat your oven to 350°F (175°C ) with a rack set in the lower third of the oven.
• Whisk together the chocolate, cocoa, and boiling water in a small bowl or measuring cup with a spout. Cover and allow to cool to room temperature. (For faster cooling, immerse the bottom of the container in an ice bath and stir to cool.)
  
• In another bowl, whisk together the egg yolks and whites, then incorporate the sour cream and vanilla.
• In a stand mixer fitter with the flat paddle, beat together the flour, sugar, baking soda, and salt to evenly mix them.
• Take the bowl off the mixer. Add the butter and the chocolate mixture to the dry ingredients and smash it in a little with the beater attachment so that when you turn the mixer on it doesn't fling flour everywhere. Reposition the bowl and mix on low until the dry ingredients are just moist.
• Raise the speed to medium and beat for 30 seconds. Use a spatula to scrape down the sides of the bowl, also scraping around the inside bottom center of the bowl.
• With the mixer on medium-low speed, add the egg mixture in 2 additions, fully incorporating after each addition. The batter will be somewhat fluffy. scrape down the sides again.
• Divide the batter into 2 prepared 9 x 2" round cake pans, 645 grams of cake batter in each. Smooth the tops of the cakes with an offset spatula. (Using parchemnt rounds in the bottoms of the pans will save you grief later on.)
• Bake the cake. Set your timer for 25 minutes. When the timer sounds, check the temp of your cake in the center using your Thermapen ONE. If the cakes are below 200°F (93°C), keep cooking, setting the timer to remind you to check again soon. If they are done, take them from the oven and set the pans on a cooling rack.
• Let the cakes cool in the pans for 10 minutes, then run a thin knife or spatula around the edges and turn the cakes out onto the cooling racks to finish cooling.

While the cake is baking and cooling, make the mousseline icing

• In a stand mixer bowl, cream the butter with the flat beater on medium-high speed for about 2 minutes, until smooth and creamy. Scrape the butter into another bowl and set aside for later. Wash the mixer bowl well.
• In the degreased mixer bowl, whip together the egg whites and cream of tartar, just until foamy.
• In a small saucepan, stir 150 g of the sugar, the orange zest, and the water together. Heat on medium heat, stirring, until the sugar dissolves. Keep on low heat.
• While the syrup comes up to temp, use the whisk attachment on the mixer to start beating the eggs, increasing the speed to medium-high. When soft peaks have formed, add the remaining 50 g sugar and beat until stiff peaks form.
• Bring the syrup to a boil and cook, checking the temperature frequently with your Thermapen ONE until the temperature reaches 248-250°F (120°C), adjusted 1°F less for every 500 ft above sea level you're cooking at.
• Turn the mixer to low speed and slowly pour the syrup into the bowl, right by the edge. Increase the mixer speed to high and beat for a few seconds, then turn the speed down to medium and beat for 2-4 minutes, letting the heat escape, until the meringue reaches 70°F (21°C)—use your Thermapen to check it periodically.
• Scrape the butter into the meringue mixture and beat on medium speed until the icing is smooth and creamy. The butter may look curdled, but keep going. The temperature should be between 68 and 70°F (20 and 21°C).
• Beat in the vanilla and the melted chocolate until the mixture is homogenous with no chocolate streaks. Scrape down the bowl and mix for one more minute.

Make your cake!

• Assemble the cakes and icing in any fashion you like, then eat it all or share it with your loved ones. Enjoy!