Preparation

Make the cake

• Position a rack in the center of the oven and heat the oven to 325°F. Coat a 13×18-inch rimmed baking sheet with cooking spray.
• Put the chocolate and cocoa in a medium heatproof bowl, add the boiling water, and whisk to combine and melt the chocolate. Set aside to cool slightly. In a small bowl, combine the flour, baking soda, and salt.
• Beat the sugar and butter in a stand mixer fitted with the paddle attachment on medium speed until light and fluffy, 3 to 5 minutes, stopping to scrape the bowl halfway through. Add the eggs one at a time, beating well after each addition. Slowly add the buttermilk and vanilla—the batter will look curdled at this point. Reduce the mixer speed to low. Alternate adding the flour mixture (in 3 additions) with the chocolate mixture (in 2 additions), ending with the flour mixture. Scrape the batter into the prepared pan and tap the pan on the counter to level the batter.
• Bake until the cake springs back when poked in the center with a finger, about 20 minutes. Cool completely in the pan on a wire rack. (Well-wrapped, the cake will keep for up to 2 days in the refrigerator, or up to 1 month in the freezer.)

Make the coconut-pecan filling

• Pulse the coconut and pecans in a food processor until they resemble coarse cornmeal.
• Combine the sugar, butter, egg yolks, evaporated milk, and vanilla in a 3-quart saucepan. Cook over medium-low heat, stirring constantly with a spoon or spatula, until the mixture is thick enough to coat the spoon and hold a trail when a finger is drawn through it, 6 to 7 minutes. Transfer to a stand mixer fitted with the whisk attachment and whip on high speed until cooled, fluffy, and slightly lightened in color, 6 to 10 minutes. Add the coconut-pecan mixture and beat on medium speed until combined, 1 minute more. Refrigerate until ready to use. (You can refrigerate the filling for up to 1 week.)

Make the chocolate mousse

• Melt the chocolate and butter in a large bowl set in a skillet of barely simmering water. Remove from the heat and stir with a spatula to combine. Set aside to cool slightly.
• Whip the egg whites in a clean stand mixer fitted with a clean whisk attachment on medium-high speed to soft peaks, 1 to 2 minutes. Slowly add the sugar and continue to whip to stiff peaks, 3 to 4 minutes.
• In a medium bowl, whip the cream with a whisk or electric hand mixer to medium-stiff peaks.
• In a small bowl, combine the egg yolks, vanilla, and salt. Mix the egg yolk mixture into the chocolate mixture with a spatula. Fold in the whipped egg whites, then fold in the whipped cream—there shouldn’t be any visible streaks.

Assemble the bombes

• Set twosilicone bombe moldsthat hold at least 5 bombes each (3-1/4-inch diameter and 1/2-cup capacity) on a 13×18-inch rimmed baking sheet.
• Turn the cake out onto a cutting board. Use a 3-1/8-inch round cutter to cut 10 circles from the chocolate cake. Reserve the remaining cake foranother use.
• Scoop about 1/4 cup of the mousse into each mold and smear it evenly all over the insides of the molds with the back of a small spoon. Refrigerate or freeze until set, 10 to 20 minutes. Evenly distribute the coconut-pecan filling among the molds (about 2-1/2 Tbs. each), and then gently press a cake circle into each mold. Cover with plastic wrap and freeze overnight.
• Unmold the frozen bombes one at a time onto a wire rack set in a rimmed baking sheet. Let sit until nearly room temperature, 2 to 3 hours.

Glaze the bombes

• Put the chopped chocolate and corn syrup in a medium stainless-steel bowl. In a small saucepan, heat the cream over medium-high heat until just boiling, then pour it over the chocolate mixture and let sit for 3 minutes. Whisk gently until smooth. Transfer to a liquid measuring cup for easy pouring.
• 将釉料均匀地倒在炸弹上，以完全涂抹它们。冷藏直到设定约2小时。（将炸弹釉提前不超过8小时。）
• Let the bombes sit at room temperature for about 30 minutes before serving.