Preheat oven to 325°F. In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add eggs, chocolate and vanilla; mix well. Combine flour and salt; stir baking soda into sour cream. Alternately add sour cream and flour mixture to butter mixture. Pour batter into prepared pan. Bake 20-25 minutes or until toothpick inserted comes out clean. Cool 10 minutes in pan on rack; remove and cool completely before decorating.