Preparation Steps

• Preheat the oven to 350°F. Butter two 9-inch round cake pans, line the bottoms with parchment circles, then butter the parchment and dust each pan lightly with flour; knock out excess flour.
• Place the semisweet and unsweetened chocolates in a heatproof bowl with the hot water (or coffee). Microwave in 25–30 second bursts, stirring between bursts, until smooth. Let the chocolate cool to warm temperature.
• In the bowl of a stand mixer fitted with the paddle attachment, beat the room-temperature butter and 1 1/8 cups sugar on medium-high until very pale and fluffy, about 4–5 minutes. Scrape down the bowl.
• With the mixer on low, pour in the cooled melted chocolate in a steady stream, then beat until incorporated. Add the egg yolks one at a time, beating briefly after each. Stir in the vanilla and espresso powder.
• Whisk together the flour, baking powder, baking soda, and salt in a medium bowl. Fold half the dry mix into the chocolate mixture, then add the buttermilk and fold, followed by the remaining dry ingredients. Mix just until combined — do not overwork.
• In a clean bowl, beat the reserved egg whites until soft peaks form. Gradually add 2 tablespoons of sugar (from the measured sugar reserve) and continue to beat to glossy stiff peaks. Fold about one-quarter of the whites into the batter to loosen it, then gently fold in the remainder in two additions until no white streaks remain.
• Divide the batter evenly between the prepared pans, smooth the tops, and bake in the center of the oven for 38–42 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs. Rotate pans halfway if your oven runs unevenly.
• Let the cakes cool in the pans for 15 minutes, then run a thin knife around the edges and invert onto racks. Peel off parchment and transfer layers to a wire rack. Chill the layers in the refrigerator for 30 minutes to firm up — chilling makes trimming and slicing easier.
• While the cakes chill, make the coconut–pecan filling. In a saucepan, whisk together the heavy cream, 1 cup sugar, and the 3 egg yolks until smooth. Cook over medium-low heat, stirring constantly, until the mixture thickens slightly and coats the back of a spoon, about 6–8 minutes. Do not boil.
• Immediately pour the hot custard into a bowl containing the butter, salt, toasted coconut, and chopped pecans. Stir until the butter melts and the mixture is uniform. Let cool to room temperature, then cover and chill briefly if it seems too loose before assembly.
• Make the rum syrup by heating the water and 3/4 cup sugar in a small saucepan just until the sugar dissolves. Remove from heat and stir in the dark rum. Cool to warm before using.
• Prepare the ganache: put the chopped chocolate, honey, and butter in a heatproof bowl. Heat the cup of heavy cream until it just comes to a simmer, then pour it over the chocolate mixture. Let sit for one minute, then whisk to a glossy ganache. Cool to room temperature and refrigerate briefly until spreadable but still pourable, about 15–25 minutes.
• Trim the chilled cake rounds if needed; using a long serrated knife, split each layer horizontally to create four thinner layers (or cut to desired thickness). Place the first layer on a serving plate and brush generously with the rum syrup.
• Spoon about 3/4 cup of the coconut–pecan filling over the first layer and spread to the edges. Repeat with the next two layers, brushing each with syrup and spreading filling. Reserve a portion of the filling to mound on the top layer.
• Pour most of the ganache around the sides of the assembled cake and use an offset spatula to smooth it; leave the top for the coconut topping. If the ganache firms too much, warm it briefly over a bowl of hot water to loosen. Pipe or spoon a decorative border of ganache on the top rim, then pile the remaining coconut–pecan filling in the center.
• Let the finished cake sit at room temperature for 30 minutes to settle, then refrigerate for at least 1 hour to set the ganache before slicing. Bring slices to room temperature before serving for best texture and flavor.