2To make the cake, in a bowl, combine the 1 cup cocoa powder and the chocolate. Add the boiling water and whisk until the chocolate melts and the mixture is smooth and blended. Set aside.
Over a sheet of parchment, sift together the flour, baking soda and salt. Set aside.
In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until smooth and creamy, 30 to 45 seconds. Reduce the speed to low, add the brown sugar and beat until blended. Increase the speed to medium and continue beating until the mixture is light and fluffy, about 5 minutes, stopping the mixer occasionally to scrape down the sides of the bowl. Add the eggs a little at a time, beating until incorporated before adding more and stopping the mixer occasionally to scrape down the sides of the bowl. Beat in the vanilla until incorporated, about 1 minute.
Reduce the speed to low and add the flour mixture in three additions, alternating with the sour cream and beginning and ending with the flour, beating just until blended and no lumps of flour remain. Slowly pour in the chocolate-cocoa mixture and beat until no white streaks are visible, stopping the mixer occasionally to scrape down the sides of the bowl. Using a rubber spatula, gently fold in the chocolate chips.
Pour the batter into the prepared pan, spreading the batter so the sides are about 1 inch higher than the center. Bake until a toothpick inserted into the center of the cake comes out with only a few moist crumbs attached to it, 60 to 65 minutes. Transfer the pan to a wire rack and let the cake cool upright in the pan for 15 minutes. Invert the pan onto the rack and lift off the pan. Let the cake cool completely, at least 1 hour.
Return the cooled cake to the pan. Using a serrated knife, gently saw off any excess cake that extends over the edge of the pan. Set the wire rack on a parchment paper lines baking sheet. Invert the pan onto the rack and lift off the pan.