- Butter two 9-inch cake pans, then line the bottoms with rounds of parchment or wax paper. Preheat the oven to 350°.
- Melt both chocolates together with the 6 tablespoons of water. Use either a double-boiler or a microwave. Stir until smooth, then set aside until room temperature.
- In the bowl of an electric mixer, or by hand, beat the butter and 1 ¼ cup of the sugar until light and fluffy, about 5 minutes. Beat in the melted chocolate, then the egg yolks, one at a time.
- Sift together the flour, baking powder, baking soda, and salt.
- Mix in half of the dry ingredients into the creamed butter mixture, then the buttermilk and the vanilla extract, then the rest of the dry ingredients.
- In a separate metal or glass bowl, beat the egg whites until they hold soft, droopy peaks. Beat in the ¼ cup of sugar until stiff.
- Fold about one-third of the egg whites 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 30 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.
- Mix the cream, sugar, and egg yolks in a medium saucepan. Put the 3 ounces butter, salt, toasted coconut, and pecan pieces in a large bowl.
- Heat the cream mixture and cook, stirring constantly (scraping the bottom as you stir) until the mixture begins to thicken and coats the spoon (an instant-read thermometer will read 170°.)
- Pour the hot custard immediately into the pecan-coconut mixture and stir until the butter is melted. Cool completely to room temperature. (It will thicken.)
- In a small saucepan, heat the sugar and water until the sugar has melted. Remove from heat and stir in the dark rum.
- Place the 8 ounces of chopped chocolate in a bowl with the corn syrup and 1 ½ ounces of butter.
- Heat the cream until it just begins to boil. Remove from heat and pour over the chocolate. Let stand one minute, then stir until smooth. Let sit until room temperature.
- Remove the cake layers from the pans and cut both cake layers in half horizontally, using a serrated bread knife. Set the first cake layer on a cake plate. Brush well with syrup. Spread ¾ cup of the coconut filling over the cake layer, making sure to reach to the edges. Set another cake layer on top.
- Repeat, using the syrup to brush each cake layer, then spreading ¾ cup of the coconut filling over each layer, including the top.
- Ice the sides with the chocolate icing, then pipe a decorative border of chocolate icing around the top, encircling the coconut topping.
- (It may seem like a lot of chocolate icing, but use it all. Trust me. You won’t be sorry.)