Cake: Grease 2 cake mold of 9 inches (23 cm) of diameter and put parchment paper in the bottom of the 2 mold. Grease the paper and flour it. Set aside. Put chocolate and butter in a bowl that goes in the microwave. Heat at medium-low (50 %) 1 to 2 minutes or until butter has melted. With a wooden spoon, stir until creamy. Pour boiling water and stir well.
In a large bowl, with an electric mixer, beat eggs and sugar until the mixture is inflated. Add vanilla, then the melted chocolate mixture. In a small bowl, mix sour cream and baking soda and let rest. In another bowl, sift the flour, cocoa, baking powder and salt. Add dry ingredients to the egg mixture, alternating with the sour cream mixture. Distribute the dough in the 2 mold.
Bake in center of preheated oven of 350F 25 to 30 minutes or until a toothpick inserted in the middle of the cakes come out clean. Put the molds on racks and let cool for 10 minutes. Unmold the cakes on the racks, remove delicately the parchment paper and let cool completely. (You can prepare the cakes in advance and wrap individually with plastic wrap. They will keep up until next day at room temperature or until 2 weeks in freezer, wrapped with a very good foil paper).
Chocolate Buttercream Frosting: In a bowl, with an electric mixer (use clean mixer), mix all ingredients until homogeneous and creamy. Set aside.
Ganache: Put chocolate in a bowl resisting to heat. In a thick bottom saucepan, mix cream and corn syrup. Heat at medium heat until bubbles begin to appear. Pour the cream mixture on the chocolat, add butter and let rest for 3 minutes. With a wooden spoon, stir until smooth. Let cool the ganache for about 15 minutes or until it has lightly thicken.
Cake Assembling: Put one cold cake on a serving plate. With a spatula, cover uniformly with all the chocolate buttercream frosting. Put the other cake on its back on the frosting. Spread uniformly the ganache on top and sides of the cake. (You can prepare the cake in advance and cover it. It will be good up to 2 days in fridge. Let return to room temperature before serving).