Preheat the oven to 350 degrees F. Prepare two 9-inch 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. Once cooled, add the buttermilk to the chocolate mixture.
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. Add the vanilla extract.
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.