Chop chocolate into small chunks and set aside. Combine cake mix, eggs, water, sour cream and oil. Blend, then beat at medium speed of electric mixer for 4 minutes. Fold in chopped chocolate. Pour into 2 greased and floured 9 inch layer pans. Bake at 350 degrees for 35 minutes, or until cake tester inserted in centers comes out clean. Cool in pans 10 minutes. Remove and finish cooling on racks.