Preheat oven to 350 degrees. Grease 2 9-inch cake pans with butter and line the bottom with a round of wax paper or parchment.
Combine chocolate and cocoa in a medium bowl. Pour the boiling water over and whisk until smooth.
Sift together the flours, baking soda and salt in a small bowl. Set aside.
Beat butter in the bowl of a stand mixer. Beat on medium-high until creamy, about 1 minute. Add brown sugar and beat on high until light and fluffy, about 3 minutes. Scrape down bowl. Increase to medium high and add the eggs one at time. Reduce speed to medium; add the sour cream and vanilla and beat until combined, about 1 minute. Scrape down bowl. On low speed add one third of flour mixture. Add one third of chocolate mixture. Repeat, ending with flour mixture. Beat just until combined. Do not over beat. Remove bowl from mixer. Scrape down sides again stirring to combine thoroughly.
Divide batter evenly in cake pans. Smooth to the edge leveling with rubber spatula.
Bake until a toothpick inserted in the center comes out clean, 20 - 23 minutes.
Cool on wire rack for 15 minutes. Run a knife or rubber spatula around cake to loosen. Invert onto plate. Peel off parchment. Reinvert. Cool completely.