Preparation Steps

• Preheat oven to 345°F. Line two 9-inch round cake pans with parchment, then grease and flour the sides. Scatter pecans and coconut on a rimmed sheet pan and toast 5–7 minutes, stirring once, until fragrant and lightly golden. Cool completely; keep oven on for the cakes.
• Make the coconut–pecan filling: In a medium saucepan off heat, whisk evaporated milk, egg yolks, brown sugar, granulated sugar, and salt until smooth. Add butter. Cook over medium-low, stirring constantly with a heatproof spatula, until the mixture thickens to a custard that coats the spatula and small bubbles form at the edges, 8–10 minutes (or about 185°F). Remove from heat; stir in vanilla, rum/bourbon, toasted pecans, and toasted coconut. Transfer to a shallow bowl, press parchment or plastic directly onto the surface, and cool to room temperature, then refrigerate until spreadable.
• Melt the chocolate: Combine hot water and espresso powder in a heatproof bowl. Add chopped German chocolate, let sit 2 minutes, then stir until smooth and glossy. Set aside to cool until just warm.
• Mix the cake batter: In a stand mixer, beat softened butter and granulated sugar on medium-high until pale and fluffy, about 3 minutes. Beat in eggs one at a time, scraping the bowl as needed, then add vanilla. In a separate bowl, whisk flour, baking soda, baking powder, and salt. On low speed, add dry ingredients in three additions alternating with the buttermilk in two additions, beginning and ending with dry. Mix just until combined. Pour in the melted chocolate mixture and blend on low until uniformly incorporated, about 20–30 seconds.
• Bake: Divide batter evenly between prepared pans, smooth tops, and tap pans gently to release air. Bake at 345°F for 30–34 minutes, or until a toothpick comes out with a few moist crumbs. Cool in pans 10 minutes, then invert onto racks, peel off parchment, and cool completely. For cleaner assembly, chill layers 15 minutes.
• Chocolate buttercream: Beat butter and cocoa together on low until smooth. Add powdered sugar in 3 additions, alternating with splashes of heavy cream, mixing on low until combined. Increase speed to medium-high and whip 2–3 minutes until light and silky, adjusting cream for a spreadable consistency. Beat in vanilla.
• Assemble: Place one cake layer on a stand. Spread half of the coconut–pecan filling over the top, stopping 1/2 inch from the edge. Chill 10 minutes to help it set, then top with the second layer. Apply a thin crumb coat of chocolate buttercream around the sides; chill 10–15 minutes. Finish the sides with a smooth final coat of buttercream. Spoon the remaining coconut–pecan filling over the top and gently nudge to the edges, leaving a small border. Slice and serve.