Ingredients
3 sticks unsalted butter, softened, plus more for pans
1 cup boiling water
3/4 cup Dutch-process cocoa powder
4 ounces bittersweet chocolate, chopped (3/4 cup)
3 1/2 cups unbleached all-purpose flour
1 teaspoon baking powder
3/4 teaspoon baking soda
1 1/2 teaspoons kosher salt (we use Diamond Crystal)
2 cups packed light-brown sugar
4 large eggs, room temperature
2 teaspoons pure vanilla extract
1 cup sour cream
1 1/4 cups granulated sugar
5 large egg whites
3 sticks unsalted butter, room temperature
9 ounces bittersweet chocolate, melted and cooled
1 cup granulated sugar
Yellow and red gel-paste food coloring
4 ounces semisweet chocolate, chopped (3/4 cup)
Pinch of kosher salt (we use Diamond Crystal)
1/2 cup heavy cream
Preparation
Cake: Preheat oven to 325°F. Butter two 9-inch round cake pans. Line bottoms with parchment rounds; butter parchment. Combine boiling water, cocoa, and chocolate in a bowl; stir until chocolate has melted. Let cool 10 minutes. Meanwhile, in another bowl, whisk together flour, baking powder, baking soda, and salt.
Beat butter with brown sugar on medium-high speed until light and creamy, 2 to 3 minutes. Beat in eggs, one at a time, blending well after each addition. Beat in vanilla, then chocolate mixture to combine. Reduce speed to low; beat in flour mixture in two additions, alternating with sour cream, until just combined.
Divide batter evenly between prepared pans, smoothing tops with an offset spatula. Bake until a tester inserted into centers comes out clean, 35 to 40 minutes. Let cakes cool in pans on a wire rack 20 minutes. Turn out cakes and place on rack, top-sides up; let cool completely. With a serrated knife, trim tops of cakes so they are level.
Buttercream: In the bowl of a mixer, whisk together granulated sugar and egg whites. Place over (but not in) a pot of simmering water; continue whisking constantly until sugar has completely dissolved and mixture is warm, 2 to 3 minutes. Place bowl in mixer fitted with the whisk attachment and beat on high speed until thick, glossy, and cool, 8 to 10 minutes.
Switch to paddle attachment. Beat in butter, 2 tablespoons at a time, on medium-high speed until combined. Beat in chocolate. Reduce speed to low and continue beating until air bubbles are gone, 1 to 2 minutes more.
Flames: Line several baking sheets with parchment. In a small saucepan, combine granulated sugar and 1/4 cup water over medium-high heat, stirring until sugar has dissolved. Bring to a boil and cook, swirling pan occasionally, until mixture registers 300° to 310° (hard-crack stage) on a candy thermometer, 6 to 8 minutes. Swirl in a drop or two of yellow food coloring, then pour syrup into a glass measuring cup.
Working quickly, pour lengths of syrup (each between 1 and 3 inches long) onto prepared sheets, dragging the tip of a knife through in random places to create flame shapes. If desired, add a drop of red food coloring to the last few tablespoons of syrup to create redder flames. Let stand until hardened, about 5 minutes.
Assembly: Place one cake layer on a stand, trimmed-side up, and spread 1 1/2 cups buttercream evenly over surface. Top with remaining cake layer, trimmed-side down. Spread a thin layer of buttercream over top and sides. Refrigerate until firm, about 30 minutes (this ensures a crumb-free finish). Spread remaining frosting evenly over top and sides of cake, keeping top as flat and level as possible.
Ganache: Place chocolate in a small heatproof bowl; add salt. Bring cream just to a boil over medium-high heat. Pour over chocolate. Let stand 10 minutes, then whisk until smooth and shiny. Let cool a few minutes. Pour half of warm ganache over center of cake, then slowly but steadily pour remaining ganache around edges, allowing it to drip down sides. Refrigerate until set, about 1 hour. (At this point, cake can be refrigerated up to 1 day; bring to room temperature before serving.)
To garnish, press candy flames into top of cake, arranging lighter flames toward back and darker flames toward front.
