Better-Than-Mom's German Chocolate Cake

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

Cut cake layers in half horizontally using a serrated bread knife. Set the first cake layer on a cake plate. Brush well with syrup. Spread 3/4 cup filling on top, making sure to reach all the way to the edges. Repeat with remaining cake layers, syrup, and filling, including the top.

Frost the sides of the cake with chocolate frosting. Spoon remaining chocolate frosting into a piping bag and pipe a decorative border around the top and bottom of the cake.