Preheat your oven to 350°F. Grease or spray two 9-inch round cake pans and line them with parchment paper on the bottom of each cake pan.
Using a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. In a separate small bowl, whisk together the sour cream, water, and vanilla.
Using your stand mixer fitted with the flat beater attachment, beat together the butter and sugar until light and fluffy. Beat in the eggs, 1 at a time, beating well after each addition. Add the flour mixture to the butter mixture in batches alternating with the sour cream mixture, beginning and ending with the flour mixture and beating until the batter is blended well.
Divide batter between prepared pans and smooth tops. Bake the cakes in middle of oven for about 25 to 30 minutes, or until a tester inserted comes out clean. Cool cake layers in pans on racks for about 10 minutes before turning out onto racks to cool completely.