Adjust the oven racks to the middle position; heat the oven to 350ยบ F. Meanwhile, grease a and line the bottom of a 13x9 pan with parchment. Combine the chocolate and cocoa in a medium bowl; pour the boiling water over and whisk until smooth. Sift together the flours, baking soda, and salt onto a large sheet of parchment paper; set aside.
Beat the butter in the bowl of a standing mixer at medium-high speed until it is creamed, about 1 minute. Add the brown sugar and beat at high speed until light and fluffy, about 3 minutes. Stop the mixer and scrape down the bowl with a rubber spatual. With the mixer at medium-high speed, add the eggs 1 at a time, beating 30 seconds after each addition. Reduce the speed to medium; add the sour cream and vanilla and beat until combined, about 10 seconds. Stop the mixer and scrape down the bowl. With the mixer at low speed, add about a third of the flour mixture, followed by about half of the chocolate mixture. Repeat, ending with the flour mixture; beat until just combined, about 15 seconds. Do not overbeat. Remove the bowl from the mixer; scrape the bottom and sides with a spatula and stir gently to thoroughly combine.
Pour the batter into the cake pan. Place the pan on the middle rack. Bake until a toothpick or skewer inserted in the center comes out clean, about 30 minutes. Cool the cake on a wire rack for 15 to 20 minutes. Run a knife around the perimeter of the pan to loosen. Invert onto a large plate; peel off the parchment paper and reinvert onto a rack. Cool completely. Sprinkle a light dusting of powdered sugar on top or apply your favorite frosting.