Preheat oven to 350°F (175°C) and grease two 9-inch round pans.
In a bowl, mix flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, whisk eggs, buttermilk, oil, and vanilla extract until smooth.
Gradually add wet ingredients to dry, mixing until just combined.
Divide batter between prepared pans and bake for 30-35 minutes, or until a toothpick comes out clean.
Let cake layers cool completely.
For coconut pecan frosting: Whisk egg yolks with granulated sugar until pale. Cook over simmering water until thickened. Stir in butter until smooth, then add evaporated milk and shredded coconut. Cool completely.
For chocolate buttercream: Beat butter until creamy, gradually add powdered sugar, milk, and vanilla extract.
Assemble cake: Place first layer on a plate, spread coconut pecan frosting over it, then add the second layer. Cover cake with chocolate buttercream and frost the top and sides.
Garnish with chopped pecans and serve.