Makes 12 cupcakes
85g unsalted butter, very soft
7/8 cups AP flour
60g ounces unsweetened chocolate, coarsely chopped
15g Dutch-process cocoa
1/4 cup hot water
7/8 cups sugar
1 tsp baking soda
1/2 tsp salt
1/2 cup buttermilk
1 tsp vanilla extract
1 egg yolk
Adjust oven rack to middle position; preheat oven to 175 degree.
Combine chocolate, cocoa powder and hot water in medium heat-proof bowl; set bowl over saucepan containing 1 inch of simmering water and stir with rubber spatula until chocolate is melted, about 2 minutes. Add 2/8 cup sugar to chocolate mixture and stir until thick and glossy, 1 to 2 minutes. Remove bowl from heat and set aside to cool.
Whisk flour, baking soda and salt in medium bowl.
Combine buttermilk and vanilla in small bowl.
In bowl of standing mixer fitted with whisk attachment, whisk eggs and yolks on medium-low speed until combined, about 10 seconds.
Add remaining cups sugar, increase speed to high, and whisk until fluffy and lightened in color, 2 to 3 minutes. Replace whisk with paddle attachment.
Add cooled chocolate mixture to egg-sugar mixture and mix on medium speed until thoroughly incorporated, 30 to 45 seconds, pausing to scrape down sides of bowl with rubber spatula as needed.
Add softened butter 1 tablespoon 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 and 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 and fold batter once or twice with rubber 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, 15 to 20 minutes. Cool cakes in pans 15 minutes, then invert onto wire rack. Cool to room temperature before frosting, 45 to 60 minutes.
Frosting recipe found here.