Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom and sides of 9- or 8-inch square pan with baking spray with flour.
In large bowl, beat cake mix, water, sour cream, butter and egg with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Stir in 1/2 cup of the chocolate chips. Spread in pan.
Bake 9-inch pan 22 to 28 minutes, 8-inch pan 27 to 33 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
In small microwavable bowl, microwave 1/2 cup of the chocolate chips and the oil uncovered on High 45 seconds, stirring every 15 seconds, until melted. Place in small resealable food-storage plastic bag; cut off tiny corner of bag. Drizzle over top of cake. Sprinkle with remaining 1/4 cup chocolate chips. Store loosely covered.