Cake:
Center the rack in the oven and preheat the oven to 350 F (180 C).
Butter and flour 2 round cake pans of 9″ (23 cm) in diameter. Set aside.

Sift together the flour, cocoa, baking soda, baking powder, and salt. Set aside.

Work with an electric mixer, the butter and sugar for 3 minutes, then add the eggs one by one, beating between each addition.
Add the vanilla and mix.

Reduce the speed of the electric mixer and add the melted chocolate, then incorporate the dry ingredients alternately with the buttermilk. Mix well, then incorporate the boiling water.

Divide the batter into two equal portions in the prepared cake pans.
Bake for 25 to 30 minutes.
Remove the cakes from the oven and let rest for 5 minutes before unmolding on a cooling rack.

Topping and icing:
Put the egg whites in the bowl of the electric mixer.

Put the sugar, cream of tartar, and water in a small saucepan and stir to combine.
Bring the mixture to a boil over medium-high heat, cover the saucepan, and boil for 3 minutes.
Uncover and boil the syrup until it reaches 242 F (116 C) on a candy thermometer.
When the syrup is around 235 F (112 C), start beating the egg whites at medium speed.
Pour the syrup at 242 F (116 C) over the egg whites, add the vanilla, and continue beating for 5 minutes or until the egg whites reach room temperature.

Assembly:
Cut each cake in half horizontally.
Place one half of the cake and crumble it.

Place a layer of cake, then ice with the meringue.
Repeat the operation one more time, ending with a layer of cake.
Cover the cake with meringue, then cover with the reserved cake crumbs.

Refrigerate the cake for about 1 hour before serving.