Preheat oven to 350°F (180°C).
In the bowl of a stand mixer fitted with the paddle attachment, beat butter, oil, and sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs and egg yolks, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, baking powder, and salt. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition.
Spray 2 tall-sided 8-inch round cake pans with baking spray with flour. Divide batter between prepared pans (about 775 grams each), smoothing tops.
Bake until a wooden pick inserted in center comes out clean, 30 to 40 minutes. Let cool in pans for 5 minutes. Remove from pans, and let cool completely on wire racks.
Level cooled cake layers, if desired. Place 1 cake layer on a serving plate, and spread 2 cups (380 grams) Chocolate Frosting on top. Place remaining cake layer on top, and spread remaining frosting on top and sides of cake as desired. Cover and refrigerate for up to 3 days.