Pre-heat oven to 350° F. Butter two 6-inch cake rounds and line with parchment paper. Set aside.
In a small bowl, combine chocolate and cocoa powder. Pour in boiling water and whisk until smooth. Set aside and let cool.
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter and brown sugar until fluffy and smooth. Add eggs one a time 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 bowl of stand mixer. Mix at low-speed until almost combined. Pour in half of chocolate mixture and mix until almost combined. Add another 1/3 of flour, following by remaining chocolate, and then end with the last 1/3 of flour. Mix between each addition and scrape down bowl as necessary.
Transfer batter into prepared cake rounds. Divide batter equally, I suggest weighting the batter. Bake for 30 minutes, rotating pan after 15 minutes. Bake until cake is set and toothpick inserted in center of cake comes out clean. Let cool in pan for a couple of minutes. Run a knife or offset spatula around the perimeter of cake to loosen. Invert. Cool to room temperature on wire rack.