Preheat the oven to 350 °F. Spray two 9‑inch round cake pans with nonstick cooking spray and lightly dust with flour. :contentReference[oaicite:1]{index=1}
In a small bowl, mash the bananas and stir in lemon juice. Set aside.
Using an electric mixer, cream the butter and sugar until fluffy, about 5 minutes.
Add the eggs one at a time, beating after each addition.
Add the crushed vanilla wafers in 3 additions and buttermilk in 2 additions (beginning and ending with wafers). Beat until combined.
Beat in the mashed bananas and vanilla extract until fully incorporated.
Divide the batter evenly between the prepared pans and bake until a toothpick inserted in the center comes out clean, about 1 hour.
Cool in the pans for 10 minutes before turning out onto racks to cool completely.
For the filling: separate the yolk from the white of 1 egg, discarding the white.
In a saucepan, whisk together sugar, flour, cornstarch, and salt. Stir in the egg yolk, the whole egg, and then the milk.
Cook uncovered over medium heat, stirring often, until the pudding thickens (5‑7 minutes). Remove from heat and stir in vanilla extract.
Cover the pudding with plastic wrap (touching the surface) so a skin doesn’t form. Chill until completely cooled (about 30 minutes).
Spread the pudding between the cake layers. Chill the cake until filling is set (30 minutes to 1 hour).
Just before serving, sprinkle the top with confectioners’ sugar.