Make the Frosting:
Melt chocolate chips according to package directions. Immediately whisk in sour cream. Scrape into a metal bowl. Refrigerate, stirring a few times, until thick enough to spread (may take up to 4 hours). Makes 31/4 cups.
Preheat oven to 350. Grease and flour two 9-inch round cake pans.
In a large bowl with electric mixer on low speed, combine all the cake ingredients and beat ingredients just until blended.
Increase mixer speed to high and beat 3 minutes, scraping down sides of bowl a few times.
Divide batter between prepared pans. Spread evenly.
Bake 25 to 30 minutes until a pick inserted in center of cakes comes out clean.
Cool in pan on rack 10 minutes. Turn out on rack, invert and cool completely.
To frost:
Place 1 cake layer on serving plate. Spread with 1 cup frosting.
Top with second cake layer. Frost sides and top with remaining frosting.
The cake layers can be wrapped well and refrigerated up to 3 days or frozen up to 3 months.
Make the frosting about 4 hours before you plan to ice the cake, so it has time to chill to spreading consistency.
(with frosting in another bowl)