• Prepare oven and cake pans: Preheat oven to 350°F. Line 3 9-inch round cake pans with parchment paper.
• Melt chocolate: Stir together chopped chocolate and hot coffee in a medium microwavable bowl. Microwave on HIGH until melted and smooth, 30 seconds to 1 minute, stirring mixture after 30 seconds. Set aside.
• Combine dry ingredients: Whisk together flour, baking soda, and salt in a medium bowl.
• Cream butter and sugars: Beat butter and sugars with a heavy-duty stand mixer on medium speed until light and fluffy, about 3 minutes. Add yolks, 1 at a time, beating just until blended after each addition.
• Add chocolate: Add melted chocolate mixture and vanilla, and beat on low speed until blended.
• Combine wet and dry mixtures: To the butter mixture, add flour mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low speed just until blended after each addition.
• Whip egg whites: Place egg whites in a separate clean bowl, and beat with an electric mixer on high speed until stiff peaks form, about 3 minutes. Gently fold into batter.
• Add cake batter to pans: Divide batter among the 3 prepared pans.
• Bake cake layers: Bake in preheated oven until a wooden pick inserted in center comes out clean, 24 to 28 minutes.
• Cool cakes: Remove pans from oven; gently run a knife around outer edge of cake layers to loosen from sides of pans. Cool in pans on wire racks 15 minutes. Remove from pans; transfer to wire racks. Discard parchment paper. Cool completely, about 1 hour.
• Frost cake layers: Press slightly warm Frosting between layers and on top and sides of cake. Let stand until frosting is completely cool and set, about 15 minutes.

FROSTING

• Prepare oven: Preheat oven to 350°F.
• Toast nuts: Bake pecans or walnuts in a single layer in a shallow pan 8 to 10 minutes or until toasted and fragrant, stirring halfway through. Cool completely (about 20 minutes).
• Cook frosting: Meanwhile, cook evaporated milk, sugar, butter, and egg yolks in a heavy 3-qt. saucepan over medium heat, stirring constantly, 3 to 4 minutes or until butter melts and sugar dissolves. Cook, stirring constantly, 12 to 14 minutes or until mixture becomes a light caramel color, is bubbling, and reaches a pudding-like thickness.
• Add coconut, vanilla, and toasted nuts: Remove pan from heat; stir in coconut, vanilla, and toasted nuts. Transfer mixture to a bowl. Let stand, stirring occasionally, 45 minutes or until cooled and spreading consistency.