Heat oven to 180°C (160°C for dark or nonstick pan). Generously grease, or spray with baking spray with flour, bottom of 33x23cm pan.
In small bowl, toss 75 grams of the chocolate chips with 1 tablespoon dry cake mix. Make cake mix as directed on box, using remaining cake mix, water, oil and egg whites. Stir in the coated chocolate chips. Reserve 1 cup of the batter. Pour remaining batter into pan. Stir chocolate syrup into reserved batter. Drop by tablespoonfuls randomly in 8 mounds in pan. Cut through batters in S-shaped curves. Turn pan one-fourth turn; repeat.
Bake 34 to 38 minutes or until toothpick inserted in center of chocolate comes out almost clean. Run knife around sides of pan to loosen cake. Cool completely, about 1 hour. Stir remaining chocolate chips into frosting. Spread frosting over top of cake; drizzle with additional chocolate syrup. Store loosely covered.