Preheat oven to 375°F (190°C). Grease two 8 or 9-inch round cake pans and line bottoms with parchment paper.,In a large bowl, mix sugar, flour, cocoa powder, baking powder, baking soda, and salt.,In another bowl, whisk together eggs, buttermilk, oil, and vanilla extract. Combine with dry ingredients.,Stir in boiling water carefully until batter is smooth (batter will be thin).,Divide batter evenly between prepared pans and bake for 25–35 minutes or until a toothpick comes out clean.,Cool cakes in pans for 5 minutes, then transfer to wire racks to cool completely.,For coconut pecan frosting: In a saucepan, combine brown sugar, sugar, butter, egg yolks, and evaporated milk. Stir constantly over medium heat until thickened.,Remove from heat and stir in vanilla, chopped pecans, and shredded coconut. Cool completely.,For chocolate buttercream: Melt butter, stir in cocoa powder. Alternately add powdered sugar and milk, beating until desired consistency is reached. Stir in vanilla.,Assemble the cake: Place one cake layer on a serving plate. Spread a thin layer of chocolate frosting and half of the coconut pecan frosting. Add second cake layer on top.,Frost the entire cake with chocolate buttercream and top with remaining coconut frosting.