1. Heat oven to 350°F. Grease and flour two 9-inch round baking pans; line bottoms with wax paper.
2. Place chocolate in small microwave-safe bowl. Microwave at MEDIUM (50%) 1-1/2 to 2 minutes or until chocolate is melted when stirred; cool slightly.
3. Stir together flour, sugar, baking soda and salt in large bowl. Add melted chocolate, water, oil, vinegar and vanilla; beat on medium speed of mixer until well blended. Pour into prepared pans.
4. Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire racks. Cool completely. Frost as desired. 10 to 12 servings.