Preheat oven to 350. Butter two 8-inch round cake pans; dust with cocoa, tapping out excess. Line bottom of each pan with a round of parchment paper; set aside. In a medium bowl, whisk together flour, cocoa, baking powder, baking soda, and salt; set aside.
In a large bowl, using an electric mixer, beat butter and sugar until light and fluffy. Add eggs and yolks, one at a time, beating well after each addition; beat in chocolate and vanilla. With mixer on low, alternately add flour mixture in three parts and buttermilk in two, beginning and ending with flour mixture.
Divide batter between prepared pans; smooth tops. Bake until a toothpick inserted in center of a cake comes out clean, 40 to 45 minutes. Cool in pans 15 minutes; run a knife around edge of each pan, and invert cakes onto a wire rack to cool completely.
Set a rimmed baking sheet upside down on a work surface. Place one cake on sheet, and spread top with 1/3 of ganache. Place second cake on top, and spread remaining ganache over top and sides of cake. Using two wide metal spatulas, carefully transfer frosted cake to a serving platter.