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.
3 sticks unsalted butter, room temperature, plus more for pans
1 cup unsweetened cocoa powder, plus more for pans
1 cup boiling water
1 cup sour cream
3 cups all-purpose flour (spooned and leveled)
3/4 teaspoon fine salt
1 teaspoon baking powder
1 teaspoon baking soda
3 3/4 cups sugar, divided
4 large eggs, plus 5 large egg whites, room temperature, divided
5 teaspoons pure vanilla extract, divided
1 tablespoon corn syrup
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. 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.
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