Step 1
Preheat the oven to 350 degrees F (175 degrees C). Grease two 9-inch cake pans line with parchment paper and grease the parchment paper.
Step 2
Place semisweet chocolate unsweetened chocolate and water in the top of a double boiler over simmering water. Stir frequently scraping down the sides with a rubber spatula to avoid scorching until chocolate is melted about 5 minutes.
Step 3
Beat 1 1/4 cups sugar and butter until light and fluffy about 5 minutes. Beat in the melted chocolate. Add egg yolks one at a time mixing well after each addition.
Step 4
Combine flour baking powder baking soda and salt in a bowl. Add 1/2 of the flour mixture to the creamed butter mixture and beat until well combined. Add buttermilk vanilla extract and the remaining flour mixture.
Step 5
Beat egg whites in a separate bowl until soft peaks form. Add 1/4 cup sugar and beat until stiff peaks form. Fold about 1/3 of the egg whites into the cake batter then fold in the remaining egg whites but do not overmix. Divide batter between the 2 prepared cake pans.
Step 6
Bake in the preheated oven until a toothpick inserted into the center comes out clean 40 to 45 minutes. Cool for 10 minutes in the pans before inverting onto wire racks to cool completely.
Step 7
Prepare the filling while the cakes are cooling. Combine cream sugar and egg yolks in a medium saucepan and mix until well combined. Cook over medium heat stirring constantly until custard begins to thicken and an instant-read thermometer inserted into the center reads 170 degrees F (76 degrees C).
Step 8
Combine coconut pecans butter and salt in a bowl. Pour hot custard over the pecan-coconut mixture and stir until butter is melted. Cool completely to room temperature.
Step 9
Make the syrup by heating water and sugar in a small saucepan over medium heat until sugar has dissolved. Remove from heat and stir in dark rum.
Step 9
Combine chopped chocolate butter and corn syrup in a bowl for the frosting. Heat cream in a saucepan until it just begins to boil. Remove from heat and pour into the bowl. Let stand for 1 minute then stir until smooth and chocolate is melted. Cool to room temperature.