The devil is in the details -- it's also in this sinfully chocolatey cake with fluffy frosting that is sure to be a hit.
Preheat oven to 350 degrees. Butter two 8-inch round cake pans and line with parchment; butter parchment and dust pans with cocoa.
Whisk boiling water into cocoa until smooth; whisk in sour cream. In a separate bowl, whisk together flour, salt, baking powder, and baking soda.
In a large bowl, using a mixer, beat butter and 2 1/4 cups sugar on medium-high until fluffy and pale yellow. Beat in whole eggs, one at a time, until incorporated. With mixer on low, add flour mixture in 3 additions, alternating with 2 additions cocoa mixture, and beat until combined. Fold in 4 teaspoons vanilla.
Divide batter among pans. Bake until a toothpick inserted in center comes out clean, 45 to 50 minutes, rotating pans halfway through. Transfer pans to wire racks and let cool completely. Remove cakes from pans and discard parchment. Trim cakes if desired.
In a heatproof bowl set over (not in) a pan of simmering water, whisk together remaining 1 1/2 cups sugar, egg whites, and corn syrup until sugar is dissolved. Remove from heat; using mixer, beat on high until stiff, fluffy peaks form, about 5 minutes. Fold in remaining 1 teaspoon vanilla.
Transfer a cake layer to a serving plate and top with 1 cup frosting. Stack second cake on top and cover top and sides with remaining frosting.
The cake layers can be made up to two days ahead; wrap cooled cakes tightly in plastic and keep at room temperature. Frost the cake up to three hours before serving.