Heat oven to 325. Lightly spray a 9" x 13" baking pan w/ cooking spray. In lg bowl, beat cake mix, butter and 1 egg on low until combined.
Spoon out 2/3 cup batter and set aside. Spread remaining batter in pan, bake for 15 mins, or until light brown. Cool 10 mins.
In lg bowl, combine reserved batter, corn syrup, brown sugar, 3 eggs and vanilla. Beat until well combined. Fold in pecans. Pour pecan mix over baked cake. Bake about 45 min, or until a toothpick inserted in center comes out clean. Cool 20 mins. Slice.