Cream butter or margarine, sugar and eggs in small bowl until fluffy. Add sour cream and vanilla. Beat well. Combine flour, cocoa, baking soda, and salt. Blend into creamed mixture. Stir in chocolate chips. Spread evenly in greased 9 inch baking pan. Bake at 350 for 30 to 35 minutes or until toothpick inserted in center comes out clean.