For the Super-Moist Chocolate Cake: In a microwave oven on Medium (50%), in a microwave-safe bowl, heat 2 ounces semi-sweet chocolate and 3 ounces unsweetened chocolage, stirring at 30-second intervals, until just melted, about 2 minutes. Let cool until tepid but still fluid.
Sift the flour, natural cocoa powder, baking soda, espresso powder, and salt in a medium bowl. In a glass measuring cup, stir together the buttermilk and 2 teaspoons vanilla. In a large bowl, beat the brown sugar and 2 sticks (16 tablespoons) butter with an electric mixer set on high speed until the mixture is pale and fluffy, about 3 minutes. One at a time, beat in the eggs and yolk, scraping down the sides of the bowl with a rubber spatula as needed. Beat in the tepid chocolate. With the machine on low speed, in thirds beat in the flour mixture, alternating with 2 equal additions of the buttermilk mixture, beating until smooth and scraping down the sides of the bowl as needed. Gradually beat in the boiling water.
Position an oven rack in the center of the oven and preheat the oven to 350°F. Line fifteen 3-by-1 ½-inch muffin cups with paper liners. Lightly butter a 9-by-2-inch round cake pan. Line the bottom with parchment paper or waxed paper. Dust the inside with flour and tap out the excess.
Using a 1/3-cup food portion scoop or a spoon, transfer the batter to the muffin cups, filling them about three-quarters full. Spread the remaining batter in the cake pan. Put the pans in the oven and bake until a wooden toothpick inserted in the center of a cupcake comes out clean, about 25 minutes. Remove the muffin pans and set on wire cooling racks. Continue baking the cake until a wooden toothpick inserted in the center comes out clean, about 5 minutes more. Set the pan on a wire cooling rack. Let cool in the pans on the racks for 10 minutes.
Remove the cupcakes from the pans and transfer to the racks. Run a knife around the inside of the cake pan to release the cake, invert onto a rack, and remove the pan and paper. Let the cupcakes and cake cool completely.
For the Chocolate Pudding Icing: In a small bowl, whisk the cornstarch into ¼ cup of the water to make a smooth paste. In a medium saucepan, whisk the remaining 1¼ cups of water with the sugar, Dutch-processed cocoa powder, and corn syrup. Bring to a boil over medium heat, stirring constantly. Whisk the cornstarch mixture again, and stir into the cocoa mixture. Reduce the heat to medium-low and simmer, stirring often, until the mixture is as thick as pudding and very dark brown, about 3 minutes. Remove from the heat and stir in 4 tablespoons butter and 1 teaspoon vanilla. Add the chocolate. Let stand for 2 minutes and stir until the chocolate is melted. Pour into a small bowl and press plastic wrap directly against the surface of the chocolate mixture. Refrigerate until chilled.
Crumble the cake well with your hands. Put into a pie plate or shallow bowl.
Stir the icing well to smooth it out. Using a 1/3-cup food portion scoop, mound icing on top of each cupcake. Gently spread the icing to the edges of each cupcake to shape the icing into a dome. Roll the edges of each cupcake in the crumbs to coat, then the top. (The cupcakes can be refrigerated for up to 1 day. Let stand at room temperature for 2 hours before serving.)