Preheat oven to 350° F. Generously grease two 8-inch round cake pans and caver the pan bottoms with rounds of parchment paper. Grease the parchment rounds and dust the pans with flour, tapping out excess. Alternatively, you can use my Miracle Cake Pan Release.
Mix the cocoa and instant espresso powder in a small bowl. Add boiling water and mix until smooth. Let cool and then stir in buttermilk and vanilla. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or using a hand mixer), beat butter on medium high speed until smooth—30 seconds. Gradually add sugar and beat until fluffy and almost white—3 to 5 minutes. Add eggs 1 at a time, beating 1 minute after each egg.
In a separate bowl, whisk together the flour, baking soda, and salt.
With the mixer on low, add about a third of the flour mixture to the batter. Follow immediately with about a third of the cocoa mixture. Repeat the process twice more.
When the batter appears blended, stop the mixer, scrape sides with a rubber spatula, and return mixer to low speed. Beat until the batter is “satiny,” about 15 seconds. Divide batter evenly between two prepared pans. Using an offset spatula, smooth the tops. Tap pans to release any air bubbles.
Bake for 23-30 minutes until they feel firm in the center and a toothpick inserted in the center comes out clean (maybe a crumb or two on it). Let cool for at least 10 minutes. Transfer to a wire rack to cool completely.
Coconut Pecan Filling
Mix the egg yolks, sugar, and salt in a medium bowl. Beat in the butter, and then gradually beat in the cream and vanilla.
Pour mixture into a medium, nonreactive saucepan and cook over low heat, stirring constantly. Continue until puffy and begins to thicken and the temperature reaches 180° F on an instant read thermometer—15 to 20 minutes.
Pour mixture into a medium bowl and cool to room temperature. Stir in pecans and coconut.
Use a long serrated knife to cut the cakes in half horizontally so that each cake forms 2 layers. Place 1 of the cake bottoms on a serving plate. Spread about 1 cup filling over the cake half. Repeat stacking and filling process with the remaining filling and cake, ending with a final layer of filling. Cut the cake into slices and serve.
Buttermilk substitution: Place 1 teaspoon of white vinegar or lemon juice in a measuring cup. Fill the rest of the measuring cup with milk up to the 1/3 cup measure for this recipe. Stir and let mixture sit for 5 minutes before using.Store German chocolate cake in an airtight container at room temperature for 3 days or in the refrigerator for 5 days. It is not necessary to chill this cake.How to freeze German chocolate cake: Place cake uncovered in the freezer for 1-2 hours to set. Then wrap the cake in plastic wrap or store in an airtight container. Freeze for up to 1 month. Defrost in the refrigerator before bringing to room temperature and serving.Recommended tools: Kitchen scale, stand mixer or hand mixer, 9-inch round cake pans, serrated knife, candy thermometer, small saucepan.