STEP 1
Preheat oven; whisk dry ingredients: Preheat oven to 350°F. Whisk together cake flour, cocoa, espresso powder, and salt.,
STEP 2
Beat egg whites: In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites on medium speed until frothy.,
STEP 3
Add cream of tartar, then sugar, and then vanilla: Add cream of tartar and increase speed to high. Continue beating while gradually adding sugar until stiff peaks form, 3 to 4 minutes. Beat in vanilla.,
STEP 4
Fold in half flour mixture; then other half: Remove bowl from mixer. Add half the flour mixture and gently fold in until streaky, then add remaining flour mixture and fold in until well incorporated.,
STEP 5
Transfer batter to tube pan: Gently spoon batter into an ungreased 9 1/2-inch tube pan with removable bottom; smooth top. Run a knife or small spatula through batter to release air bubbles.,
STEP 6
Bake: Bake until a tester inserted into center comes out clean, about 40 minutes.,
STEP 7
Invert, cool, and remove from pan: Invert pan; let cake cool completely in pan. Run a knife around the inner and outer perimeter of the pan to loosen. Use knife to release cake from bottom of pan and remove.,
STEP 8
Add chocolates to bowl for frosting: Place both chocolates in the bowl of a stand mixer; set aside.,
STEP 9
Heat cream and corn syrup: In a small saucepan, bring cream and corn syrup to a boil.,
STEP 10
Pour hot mixture over chocolate: Pour over chocolate; let stand 2 to 3 minutes.,
STEP 11
Whisk chocolate mixture: Whisk, starting in center and working outward, until chocolate is smooth. Let cool at room temperature, whisking occasionally, about 30 minutes.,
STEP 12
Add butter: Attach bowl to stand mixer fitted with the whisk attachment and beat on medium speed, gradually adding butter, until frosting is smooth and silky.,
STEP 13
Split cake: Using a serrated knife, split cake in half horizontally.,
STEP 14
Spread frosting on bottom half: Place bottom layer on a cake plate. Measure 2 cups frosting and spread evenly with an offset spatula.,
STEP 15
Add top layer and frost: Top with second layer. Use remaining frosting to cover cake completely, using spatula to create a smooth finish or create swirls.,
STEP 16
Garnish with chocolate: Garnish with chocolate shavings, if desired. Serve immediately.