Preheat oven to 350 degrees. Butter two 9-by-2-inch round cake pans; line bottoms with parchment paper. Butter parchment; dust with cocoa powder, tapping out excess. In a medium bowl, whisk cocoa with hot water until smooth. Whisk in sour cream; let cool. Into a medium bowl, sift together flour, baking soda, and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, 3 to 4 minutes. Add eggs, one at a time, beating to combine after each; scrape down sides of bowl as needed. Beat in vanilla. With mixer on low speed, add flour mixture in two parts, alternating with cocoa mixture and beginning and ending with flour; beat until combined.
Divide batter between prepared pans; smooth with an offset spatula. Bake until a cake tester inserted in centers comes out clean, 45 to 50 minutes. Transfer pans to a wire rack to cool 15 minutes. Invert cakes onto rack; peel off parchment. Turn cakes over; let cool completely, topside up. Transfer half of Chocolate Ganache (3 1/2 cups) to clean bowl of an electric mixer; set aside remaining. Let cool completely, stirring frequently, about 40 minutes. Attach bowl to mixer fitted with whisk attachment. Beat on medium-high speed until ganache holds soft peaks, 5 to 7 minutes.
Using a serrated knife, trim tops of cake layers to make level. Transfer one of the layers to a cake turntable or platter, and spread top with 1 1/2 cups whipped ganache. Top with remaining layer, cut side down, and spread remaining whipped ganache in a thin layer over entire cake, covering completely. Refrigerate until set, about 30 minutes.
Transfer cake to a wire rack set over a rimmed baking sheet. Pour reserved ganache over top, letting it run down the sides. If necessary, use a large offset spatula to spread from the center toward the edges, so that the cake is evenly and completely covered. Refrigerate until ganache has just begun to set, about 30 minutes. Transfer cake to a serving plate. Serve immediately, or refrigerate, covered with a cake dome, for up to 2 days.