For the chocolate cake batter:
Preheat the oven at 350 degrees F. Butter 2 (9-inch round by 2-inch deep cake pans), and line the bottom with a round of parchment paper.
Beat the 12 egg yolks with the sugar until thick and pale, about 3 minutes using a hand or stand mixer. Sift and fold in the cocoa using a spatula.
Beat the egg whites until firm peaks form. Slowly add the egg whites to the chocolate mixture in 3 increments.
Divide the batter evenly between the pans lined with parchment paper and bake in the middle of the oven for 20 minutes, or until it springs back with the touch of your fingers. Let cool slightly and unmold. Cool completely.
For the raspberry filling:
In a small saucepan, melt the raspberry jam, transfer to a bowl and add the fresh raspberries. Stir delicately to coat the raspberries with the jam. Keep aside.
For the chocolate mousse:
In a saucepan, bring the cream to a boil and remove from heat. Pour the hot cream on the chopped chocolate and let stand for 2 minutes. Whisk the ganache until smooth. Add the butter and let cool at room temperature for about 30 minutes.
Beat the 3 egg whites to soft peaks. Add the sugar and continue beating until firm peaks form. Slowly add the meringue in 3 increments to the chocolate ganache. Keep the mixture in the refrigerator for 1 hour.
To assemble the cake:
Remove the parchment paper from the cake and invert one layer onto a plate. Spread half of the chocolate mousse and garnish with the reserved raspberry mixture. Place the second cake layer on top. Spread the remaining mousse and garnish with fresh raspberries. Top with chocolate shavings and sprinkle with powdered sugar.