Preheat the oven to 350 degrees. Prepare two round cake pans by cutting circles of parchment paper to line the bottoms, then lightly spray the insides of the pans with cooking spray.

Combine the cocoa powder and finely chopped chocolate in a large bowl. Pour the boiling water over the chocolate and stir until the chocolate is completely melted, then add in the vanilla. Let this cool for a few minutes before stirring in the buttermilk.

In the large bowl of a stand mixer, beat the butter and oil together for 1 minute. Add the dark brown and granulated sugars and beat for another 2-3 minutes, until light and creamy. Scrape down the sides of the bowl and add in the eggs, one at a time, beating well after each addition and scraping down the sides of the bowl.

In a separate bowl, sift together the flour, baking soda and salt. Add half of this to the batter and mix on low speed just until combined. Pour in half of the chocolate-buttermilk mixture and mix again, just until combined, then repeat with the flour and the remaining chocolate-buttermilk mixture, scraping down the sides of the bowl between additions, until everything is incorporated.

Divide the batter between the two prepared cake pans and bake for 25 to 30 minutes, or until a toothpick or cake tester comes out clean. Be sure not to overbake.

Transfer the cakes to wire racks and allow them to cool completely before assembling the cake with frosting.

In a large bowl, cream the butter for 2 minutes using an electric mixer. Add the powdered sugar and cocoa powder and mix again at the lowest speed until combined.

Add the salt, vanilla and milk or cream and increase the speed to medium, mixing until the chocolate buttercream is light and fluffy, about 3 minutes. For a softer frosting, increase the milk or cream by small increments., half a tablespoon at a time. Makes enough to very generously frost a two-layer cake.