Heat oven to 350°F. Spray bottom of 13x9- inch pan with cooking spray. Make and bake cake mix in pan as directed on box, using water, oil and eggs. Cool. In 2-quart saucepan, mix sugar, cornstarch and salt. Add milk and yolks. Cook and stir over medium heat until mixture boils. Boil 1 minute. Stir in butter and vanilla. Remove from heat. Cover surface of pudding with plastic wrap; cool.