Preheat oven to 350°F (175°C). Butter and line two 9-inch cake pans with parchment paper.

Melt bittersweet and unsweetened chocolate with water using a double boiler or microwave. Stir until smooth and let cool.

Cream butter and 1 1/4 cup sugar until light and fluffy.

Beat in melted chocolate, then egg yolks one at a time.

Sift flour, baking powder, baking soda, and salt.

Mix half the dry ingredients into the creamed mixture, then buttermilk and vanilla, then the remaining dry ingredients.

In a separate bowl, beat egg whites until soft peaks form. Gradually add remaining 1/4 cup sugar and beat until stiff.

Gently fold one-third of the egg whites into the batter to lighten, then fold in the remaining egg whites until just combined. Do not overmix.

Divide batter into prepared pans, smooth tops, and bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean.

Cool cake layers completely.

For the filling: Mix cream, sugar, and egg yolks in a saucepan. Cook, stirring constantly, until thickened and coats a spoon.

Pour hot custard over butter, salt, and coconut in a bowl. Stir until butter melts. Refrigerate overnight.

Add pecans to the filling when ready to assemble the cake.

For the syrup: Heat sugar and water in a saucepan until sugar dissolves. Remove from heat and stir in rum.

For the icing: Place chopped chocolate, corn syrup, and butter in a bowl.

Heat cream until just boiling. Pour over chocolate mixture. Let stand for a minute, then stir until smooth. Refrigerate overnight.

Bring icing to room temperature and whip until spreadable.

To assemble: Cut each cake layer in half horizontally.

Place first layer on a plate, brush with syrup, spread a thin layer of frosting, and 3/4 cup of coconut filling to the edges.

Repeat with remaining layers, syrup, frosting, and filling.

Ice the sides with chocolate icing and pipe a decorative border around the top.