Prepare a large bowl for an ice bath. Be sure that the medium bowl will fit inside.
In a medium pot, combine the cream and cocoa and heat until a full boil forms. Remove from heat and pour over chocolate pieces and butter. Wait five minutes for the milk to melt the chocolate then slowly whisk until the chocolate and butter are completely melted, blended and smooth.
Add in the vanilla and salt, then place the bowl into the prepared ice bath and whisk until the chocolate has come to room temperature. Lumps will form if you stop mixing. Once cool, remove from the ice bath and set aside.
Whip the egg whites with a whisk attachment in a mixer. Once the whites are foamy, slowly add in the sugar and continue whipping until stiff peaks form.
Fold in a third of the chocolate into the meringue then gradually add in all of the chocolate and fold until the chocolate and meringue are incorporated.
Reserve a cup of the frosting.
To assemble the cake
Place the top layer of the cake, upside down into a springform pan. Add ⅓ of the chocolate mousse frosting allowing it to run over the sides. Place the second slice of cake on top along with more frosting and finally top with the bottom slice of the cake with the bottom up (smooth side up). Cover the cake and fill in the sides with the frosting. Allow to chill overnight.
Slide a warm, dry knife along the edge of the springform pan and slowly release the clasp. Remove from the pan and place on a serving dish.
* If you can safely remove the bottom of the pan, do so. I used a Large Cake Lifter to move my cake off the springform bottom and onto the cake platter.
Use the extra frosting to touch up any spots and add it to the top of the cake. Use a serrated knife to make groves in the top of the cake or you can use an inexpensive tool like this Decorating Comb.
Take the chopped macadamia nuts and gently press them into the sides of the cake. A lot will fall off onto the platter. Just keep going around until the cake sides are coated.
Finally, top with the truffles and place in the fridge for an hour before serving. I found this cake was best served cold.