• Preheat oven to 350°F. Generously butter two 9-inch round cake pans and line with circles of parchment paper, then generously butter parchment. Be sure your pans are VERY well coated with butter or non-stick spray.
• In a large bowl, stir together flour, baking powder, baking soda, and salt. In a separate large mixing bowl, beat butter, oil, and sugar with an electric mixer at medium speed until light and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well and scraping down the bowl after each addition. Beat in egg yolks. At low speed, beat in sour cream and milk until just combined. Add flour mixture in three batches, mixing until each addition is just Incorporated.
• Spread batter evenly in cake pans, smoothing the tops with a spatula. Bake until firm and golden; or until a cake tester inserted in center of cake comes out clean; about 40 minutes.
• Allow cakes to cool in pans for 10 minutes, then run a knife around edge of pan, loosing any crumbs that may be stuck. Invert onto rack and discard parchment, then cool completely, about 1 hour.

For Nutella Buttercream:

• In the body of a stand mixer fitted with the paddle attachment beat butter until light and fluffy. Gradually add the sugar, then cocoa powder, and beat until smooth and creamy. Add 3 tablespoons of the half and half and beat until smooth. Fold in Nutella, and add additional powdered sugar (a 1/4 cup at a time) if the frosting looks too thin. Add additional half and half if the frosting looks too thick, a tablespoons at a time. Spread evenly over cake.