Ingredients
- Frosting:
- 12 Tbsp. (1 1/2 sticks) unsalted butter, very soft (plus extra for greasing pans)
- 1 3/4 c. unbleached all-purpose flour (plus extra for dusting pans)
- 4 oz. unsweetened chocolate, coarsely chopped
- 1/4 c. cocoa
- 1/2 c. hot water
- 1 3/4 c. sugar
- 1 1/2 tsp. baking soda
- 1 tsp. salt
- 1 c. buttermilk
- 2 tsp. vanilla
- 4 large eggs
- 2 large egg yolks
- 16 oz. semisweet chocolate, finely chopped
- 8 Tbsp. (1 stick) unsalted butter
- 1/3 c. sugar
- 2 Tbsp. corn syrup
- 2 tsp. vanilla
- 1/4 tsp. salt
- 1 1/4 c. heavy cream (cold)
Preparation
Step 1
Heat oven to 350*. Grease and flour 2 round cake pans. Combine chocolate, cocoa, & water in medium heatproof bowl; set bowl over saucepan with 1 inch simmering water and stir with spatula until chocolate is melted, about 2 minutes.
Add ½ c. sugar to chocolate mixture and stir until thick and glossy, 1-2 minutes. Remove bowl from heat and set aside to cool.
Whisk flour, baking soda, & salt in medium bowl. Combine buttermilk & vanilla in small bowl. In bowl of standing mixer fitted with whisk attachment, whisk eggs & yolks on medium-low speed until combined, about 10 seconds. Add remaining 1 ¼ c. sugar, increase speed to high, and whisk until fluffy & lightened in color, 2-3 minutes.
Replace whisk with paddle attachment. Add cooled chocolate mixture to egg/sugar mixture and mix on medium speed until thoroughly incorporated, 30-45 seconds, pausing to scrape down sides of bowl with spatula as needed. Add softened butter 1 Tbsp. at a time, mixing about 10 seconds after each addition. Add about 1/3 of flour mixture followed by 1/2 of buttermilk mixture, mixing until incorporated after each addition (about 15 seconds).
Repeat using half of remaining flour mixture & all of remaining buttermilk mixture (batter may appear separated). Scrape down sides of bowl and add remaining flour mixture; mix at medium-low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer & fold batter once or twice with spatula to incorporate any remaining flour.
Divide batter evenly between prepared cake pans; smooth batter to edges of pan with spatula. Bake cakes until toothpick inserted into center comes out with a few crumbs attached, 25-30 minutes. Cool cakes in pans 15 minutes, then invert onto wire rack. Cool cakes to room temperature before frosting, 45-60 minutes.
FROSTING: Melt chocolate in heatproof bowl set over saucepan containing 1 inch barely simmering water, stirring occasionally until smooth. Remove from heat & set aside. Meanwhile, heat butter in small saucepan over medium-low heat until melted. Increase heat to medium; add sugar, corn syrup, vanilla, & salt and stir with heatproof spatula until sugar is dissolved, 4-5 minutes. Add melted chocolate, butter mixture, & cream to clean bowl of standing mixer and stir to thoroughly combine. Place mixer bowl over ice bath & stir mixture constantly with spatula until frosting is thick & just beginning to harden against sides of bowl, 1-2 minutes (frosting should be 70*). Place bowl on standing mixer fitted with paddle attachment and beat on medium-high speed until frosting is light & fluffy, 1-2 minutes. Stir with spatula until completely smooth.