Preheat oven to 350 degrees. Grease and flour a 13 x 9 inch oblong pan. Measure all cake ingredients into large mixing bowl. Mix 1/2 minute on low speed, scraping bowl constantly. Beat 3 minutes at high speed, scraping bowl occasionally. Pour into pan and bake 40 to 45 minutes.
FROSTING: Mix butter and chocolate thoroughly. Blend in sugar. Stir in sour cream and vanilla. Beat until smooth.