Prepare the Baking Pans: Preheat your oven to 375°F (190°C). Grease two round cake pans, either 8 or 9 inches in diameter. For easy cake removal, cut parchment paper circles to fit the bottoms and place them inside the pans.
Mix Dry Ingredients: In a large mixing bowl, sift together the granulated sugar, all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Stir well to combine all dry components evenly.
Combine Wet Ingredients: In a separate bowl, whisk the eggs thoroughly, then add the buttermilk, vegetable oil, and vanilla extract. Mix until the ingredients are fully blended.
Combine Wet and Dry Mixtures: Pour the wet mixture into the dry ingredients. Stir gently until just combined, taking care not to overmix. The batter will be thick at this stage.
Add Boiling Water: Slowly pour in the boiling water while stirring carefully. The batter will thin significantly—this is normal and helps create a moist cake texture.
Bake the Cake Layers: Divide the batter evenly between the prepared pans. Place them in the preheated oven and bake for 25 to 35 minutes. The exact time depends on the pan size; 9-inch pans generally require less time. The cake is done when a toothpick inserted into the center comes out mostly clean with a few moist crumbs.
Cool the Cake: Let the cakes rest in the pans for about 5 minutes, then carefully invert them onto wire racks to cool completely before frosting.
Make the Coconut-Pecan Frosting: In a medium saucepan, combine the light brown sugar, granulated sugar, butter, egg yolks, and evaporated milk. Cook over medium heat, stirring constantly to prevent burning, until the mixture reaches a gentle boil and thickens—usually several minutes. Remove from heat and stir in the vanilla extract, chopped pecans, and shredded coconut. Allow this frosting to cool fully before spreading.
Prepare the Chocolate Buttercream Frosting: Melt the butter in a medium bowl. Whisk in the cocoa powder until smooth. Gradually add powdered sugar alternated with evaporated milk, beating until the frosting achieves a spreadable consistency. Adjust thickness by adding a bit more milk or powdered sugar as needed. Stir in the vanilla extract last.
Assemble the Cake: Place one cake layer on your serving plate or cake stand. Spread a thin layer of chocolate buttercream on top. Then, evenly spoon half of the cooled coconut-pecan frosting over the chocolate layer, smoothing it gently but leaving about a ½-inch border near the edges. Place the second cake layer on top.
Final Frosting and Decoration: Cover the entire cake with the remaining chocolate buttercream frosting, smoothing it evenly around the sides and top. Finish by spreading the remaining coconut-pecan frosting generously over the top of the cake.