Place a rack in the center of the oven and preheat the oven to 350° F.
Generously grease two 9-inch round cake pans with solid vegetable shortening, then dust with flour.
Shake out the excess flour.
Set the pans aside.
Place the cake mix, sour cream, water, oil, eggs, and vanilla in a large mixing bowl.
Blend with an electric mixer on low speed for 1 minute.
Stop the machine and scrape down the sides of the bowl with a rubber spatula.
Increase the mixer speed to medium and beat 2 minutes more, scraping down the sides again if needed.
The batter should look well combined.
Divide the batter between the prepared pans, smoothing it out with the rubber spatula.
Place the pans in the oven side by side.
Bake the cakes until they spring back when lightly pressed with your finger, 28 to 32 minutes.
Remove the pans from the oven and place them on wire racks to cool for 10 minutes.
Run a dinner knife around the edge of each layer and invert each onto a rack, then invert again onto another rack so that the cakes are right side up.
Allow to cool completely, 30 minutes more.
Meanwhile, prepare the Chocolate Sour Cream Frosting.
When frosting is ready Place one cake layer, right side up, on a serving platter.
Spread the top with frosting.
Place the second layer, right side up, on top of the first layer and frost the top and sides of the cake with clean, smooth strokes DIRECTIONS FOR CAKE TOPPING AND FILLING . Melt the butter in a small saucepan over low heat or melt it in a large glass bowl in the microwave oven on high power for 45 seconds.
Place the melted butter in a large mixing bowl and add the cocoa powder.
Whisk until smooth.
Add the confectioners' sugar alternately with the sour cream, blending with an electric mixer on low speed for 1 to 1½ minutes, or until incorporated.
Stop the machine and add the vanilla. Increase the mixer speed to medium and beat until light and fluffy, 1 minute more.
Use the frosting to frost the top and sides of the cake Store this cake in a cake saver or under a glass dome, in the refrigerator for up to 1 week .Or freeze it, in a cake saver, for up to 6 months.
Thaw the cake overnight in the refrigerator before serving.