Heat oven to 350°F. Coat two 9 x 2-inch round cake pans with nonstick cooking spray. Line bottoms of pans with waxed paper; coat paper with cooking spray. Mix flour, cocoa powder, baking powder, baking soda and salt in bowl. Melt chocolate in small saucepan over low heat. Beat together butter and sugar on medium-high speed in large bowl for 2 to 3 minutes or until smooth and creamy. Add eggs, one at a time, beating well after each addition. Add vanilla and cooled chocolate; beat until combined. On low speed, beat in flour mixture alternately with milk in three additions, beginning and ending with flour; beat on medium speed for 2 to 3 minutes or until well blended. Evenly divide batter between two prepared pans.