Preheat the oven to 350°. Butter two 9-inch cake pans, then line the bottoms with rounds of parchment paper.
Melt both chocolates together with the 6 tablespoons of water in a small bowl. Use either a double-boiler or a microwave. I used a microwave on 50% for 30 seconds to 1 minute. Stir until smooth, then set aside to cool to room temperature.
In the bowl of an electric mixer beat the butter and 1 cup of the sugar on medium-high speed until light and fluffy, about 5 minutes. Add the melted chocolate and beat on medium speed until combined. Then beat in the egg yolks (yolks only), one at a time, beating well after each addition.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt.
Over low speed, beat half of the dry ingredients (the flour mixture) into the creamed butter mixture until just combined. Add the buttermilk and the vanilla extract until combined and then beat in the rest of the flour mixture.
In a separate bowl, beat the reserved egg whites on medium-high speed until they hold soft, droopy peaks. Beat in the ¼ cup of reserved sugar (slowly) and beat until they hold stiff, glossy peaks.
With a spoon, fold about one-third of the egg white mix into the cake batter to lighten it, then fold in the remaining egg whites just until there’s no trace of egg white visible.
Divide the batter into the 2 prepared cake pans, smooth the tops, and bake for about 45 minutes, until a toothpick inserted into the center comes out clean. Cool cake layers completely. While the cakes are baking and cooling, make the filling, syrup, and icing.