Heat the oven to 350 degrees. Grease and flour 2 8-inch round baking pans; set aside. In a large mixer bowl add the vegetable oil, egg and 2 teaspoons vanilla extract. Beat at medium speed until well mixed (1 minute). Add the brown sugar and granulated sugar; continue beating until well mixed (1 minute). Add the melted chocolate, 1 cup of the flour, 2/3 cup of the cocoa powder, the baking soda and salt. Beat at low speed until mixed. Add 1/2 cup of the water; beat at low speed until mixed. Add the remaining 1 cup flour and 1/2 cup water; beat at low speed until mixed, scraping the sides of the bowl as necessary. Beat at medium speed for 2 minutes. Beat in 1 cup of the sour cream until mixed. Pour the batter evenly into the pans. Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean. Remove the cake pans from the oven and cool in the pans for 10 minutes. Remove the cakes from the pans; cool completely. Meanwhile, in a medium bowl, beat the butter at high speed until creamy (about 1 minute). Add 2/3 cup cocoa powder, 1 teaspoon vanilla and 1 cup powdered sugar. Beat the frosting at low speed until mixed. Add the remaining 2 cups powdered sugar, 1/4 cup sour cream and milk; beat at low speed until mixed, scraping the sides of the bowl as necessary. Beat at medium speed until smooth and creamy. Place one cake, rounded side down, on the serving plate. Spoon and spread 1 cup of the frosting over the cake. Place the second cake, rounded side up, over the frosting. Frost the sides and top of the cake with the remaining frosting. Garnish if desired.