Base:
18 1/4 ounces (1 box) butter pecan cake mix
(set aside 1 cup of dry cake mix from package)
1/4 cup butter, softened
1 egg, slightly beaten

Topping:
3 eggs
3/4 cup packed brown sugar
3/4 cup dark corn syrup
1 teaspoon vanilla
2 cups coarsely chopped pecans

Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan; set aside.
Set aside second measure of the cake mix for the filling.
For the base, in a large mixing bowl combine cake mix, butter, and slightly beaten egg. Stir with a fork until crumbly. Turn into prepared pan. With lightly floured hands, press evenly onto the bottom to form a crust.
Bake for 12 minutes.
Meanwhile, for filling, in a medium bowl combine the second measure of eggs, brown sugar, corn syrup and vanilla. Add the reserved cake mix; stir with a fork just until blended. Some tiny cake clumps will remain. Spread filling evenly over baked crust; sprinkle with pecans.
Bake for 25 - 30 minutes or until filling appears set when pan is gently shaken. Cool completely on wire rack. Cut into bars.
Cover and chill to store.

Note:
Chocolate cake mix is also good for this recipe.