Heat oven to 350°F. Grease and flour 13x9-inch baking pan; set aside.
Combine sugar and butter in bowl; beat at medium speed, scraping bowl often, until creamy. Add sour milk, melted chocolate, eggs and vanilla; continue beating until smooth. Add flour, baking soda and salt; beat at low speed until well mixed. Add boiling water; continue beating until smooth.
Pour batter into prepared pan. Bake 40-45 minutes or until toothpick inserted in center comes out clean. Dust cake with powdered sugar or serve with dollop of whipped cream, if desired.
To sour milk, place 1 1/2 teaspoons vinegar plus enough milk to equal 1/2 cup in small bowl; let stand 5 minutes.