Martha's Perfectly Chocolate Cake

# 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. In 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