Yield: 6-inch layer cake
Preheat oven to 350 degrees F. Butter two 6-inch round cake pans and line with parchment paper. Set aside.
In a small bowl, combine unsweetened chocolate, cocoa powder and boiling water. Whisk until smooth. Set aside and allow to cool. It is ready to use when it is no longer hot, and just warm to the touch.
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter and brown sugar until fluffy and smooth. Add eggs and mix until incorporated. Add sour cream and vanilla and continue to mix.
In a medium bowl, whisk together all-purpose flour, cake flour, baking powder and salt. Add 1/3 of flour mixture into the bowl of stand mixer. Mix at low speed until almost combined. Pour in half of the cooled chocolate mixture and mix. Add another 1/3 of flour, followed by the remaining half of chocolate. End with the last 1/3 of flour. Mix between each addition and scrape down bowl as needed to ensure thorough mixing.
Divide batter evenly between prepared cake pans. Bake for 30 minutes, rotating pans after 15 minutes. Bake until cake is set and toothpick inserted in center of cake comes out clean. Let cake cool in pan for 5 minutes. Run a knife or mini offset spatula around the perimeter of the cake to loosen it from the pan. Invert and let cake cool to room temperature on a wire rack.
In the bowl of a stand mixer fitted with a paddle attachment (or using a hand mixer), cream butter until smooth. Sift together powdered sugar and cocoa powder. Add to bowl in three additions. Mix on low speed after each addition. Add salt and milk. Mix until combined. Adjust consistency according to preference. If buttercream is too thick, add milk 1 Tbsp at a time until desired consistency is achieved. If buttercream is too thin, add 1/4 cup sifted powdered sugar (up to 3/4 cup). Set aside until ready to use.
Place chopped chocolate in a large bowl. Bring heavy cream to a simmer. Pour hot cream over chopped chocolate and cover bowl with plastic wrap. Let sit for 3 minutes. Remove plastic wrap and stir ganache until homogenous. Allow to sit at room temperature for 8-10 minutes until ganache has slightly thickened.
Level cake rounds. Slice each cake round into two layers, for a total of 4 layers of cake. Build cake and frost with buttercream. Allow to chill in the fridge while ganache thickens. Once ganache is ready, pour melted chocolate over chilled cake. Immediately decorate the top of the cake with you desired toppings (Oreos, chocolate wafers, chocolate bars, chocolate candies). Place in fridge for 10 minutes to allow chocolate to set up. Remove from fridge and serve immediately.