1Cupcakes: Preheat oven to 350F. Brush standard muffin tins with butter; dust with cake flour, tapping out excess. Whisk together cake flour, baking soda and salt.
2With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy.
3Add eggs one at a time, beating until each is incorporated, scraping down sides of bowl as needed.
4Beat in vanilla. Reduce speed to low.
5Add flour mixture in three batches, alternating with two additions of buttermilk, and beating until combined after each.
6Beat in chocolate until combined.
7Divide batter evenly among prepared cups, filling each three-quarters full. Bake rotating tins halfway through, until a cake tester inserted in the centers comes out clean, 20 minutes.
8Transfer tins to wire racks to cool for 10 minutes. Run a small offset spatula or knife around edges to loosen; turn out cupcakes onto racks and let cool completely.
9To finish, use a serrated knife, and a gentle sawing motion, to split cupcakes in half horizontally. Spread a heaping tablespoon of frosting on top of each bottom half; replace top halves and spread more frosting on top of each cupcake.
10FROSTING: Combine egg yolks, evaporated milk, and brown sugar in a saucepan.
11Add butter and cook over medium heat, stirring constantly, until thick, about 10 minutes. Pour through a fine sieve into a bowl.
12Stir in vanilla, salt, coconut and pecans. Let cool completely.