For the ultimate chocolate lover, make this rich, dense chocolate cake with an equally intense, chocolatey frosting.
Preheat oven to 350 degrees. Butter two 9-by-2-inch round cake pans; line bottoms with parchment paper rounds. Butter parchment; dust with cocoa powder, tapping out excess.
In a medium bowl, whisk together cocoa and hot water until smooth. Whisk in sour cream; let cool, then whisk in chocolate. Into a medium bowl, sift together flour, baking soda, and salt.
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 addition; scrape down sides of bowl as needed. Beat in vanilla. With mixer on low speed, add flour mixture in two parts, alternating with the cocoa mixture and beginning and ending with the flour; beat until combined.
Divide batter between prepared pans (or muffin tins lined with paper liners); smooth with an offset spatula. Bake until a cake tester inserted in center comes out clean, 45 to 50 minutes (18 to 20 minutes for cupcakes).
Transfer pans to a wire rack to cool 15 minutes. Invert cakes onto rack; peel off parchment. Re-invert cakes; let cool completely, top sides up.
Using a serrated knife, trim tops of cake layers to level. Transfer a cake layer to a cake turntable or platter to frost. Using an offset spatula, spread 3/4 cup frosting on top. Set remaining cake layer into place.
Spread remaining frosting over entire cake, swirling to coat. Serve immediately, or cover with a cake dome and refrigerate; let sit at room temperature for 20 minutes before serving.