Preheat over to 350.  Spray a 13 x 9 inch baking pan with nonstick baking spray with flour.

Prepare the cake mix according to package directions for whole egg recipe.  Add mashed banana, beat at medium speed until combined.  Pour batter into prepared pan, smoothing top with an offset spatula.

Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes.  Let cool 5 minutes.  Using the handle of a wooden spoon, poke holes in the warm cake.

In a medium saucepan, combine sugar, flour, and salt; whisk in milk.  Bring to a simmer over medium heat, stirring constantly.  Remove from heat.

In a medium bowl, place egg yolks.  Whisk in half of the hot milk mixture.  Add egg yolk mixture to remaining milk mixture in saucepan whisking to combine.  Cook over medium heat until thick and bubbly, 3 to 5 minutes, stirring constantly.  Remove from heat, stir in vanilla.  Let cool 10 minutes, stirring occasionally.  Pour over cake.  Lightly cover, and refrigerate until chilled, approximately 4 hours.

Uncover, top with sliced bananas, and spread with whipped cream.  Sprinkle with vanilla wafers and pecans.  Serve immediately or cover and refrigerate up to 8 hours.