For the cake

1. Position racks in the bottom and top thirds of the oven and heat the oven to 350°F. Butter three 9×2-inch round cake pans and line each with a parchment round. Butter the parchment, then dust with flour and knock out the excess.
2. In a large, heavy saucepan, combine the butter, oil, chopped chocolate, and 1 cup water. Heat over medium heat until melted and fully combined.
3. In a large bowl, whisk the flour, sugar, and cocoa powder. Pour the hot chocolate mixture into the sugar mixture and whisk until combined.
4. Whisk in the eggs, one at a time (making sure to keep whisking – you don’t want the eggs to scramble), then whisk in the buttermilk, vanilla, baking soda, and salt. Divide the batter evenly among the prepared pans.
5. Set two pans on the top rack and the third on the lower rack. Stagger the pans on the oven racks so that no pan is directly over another. Bake, swapping and rotating the pans’ positions after 20 minutes, until a toothpick inserted in the center of each cake comes out clean, 35 to 40 minutes. Cool on racks for 10 minutes. Invert the cakes onto the racks, remove the parchment, and cool completely.

For the frosting

1. In a large mixing bowl, cream butter.
2. Gradually beat in confectioners’ sugar, cocoa and vanilla.
3. Add enough milk until frosting reaches spreading consistency.