Heat the oven to 350 F. Grease and flour a 9-by-13-inch baking pan.
Prepare the cake mix as directed with water, eggs, and oil. Spread the cake in the prepared pan and bake for 32 to 38 minutes, or as directed on the package.
Transfer the cake to a rack to cool for 10 minutes, then poke holes evenly over the whole cake. I use a wooden spoon handle.
Prepare the banana cream pudding. In a large bowl, mix the banana cream pudding with cold milk and beat with an electric mixer until well blended.
Immediately pour the pudding over the cake, letting it fill the holes you made. Spread it with a spatula to ensure some pudding gets into all the holes.
Chill the cake for at least 1 hour.
Spread the whipped topping over the cake and sprinkle with crumbled vanilla wafers.
Serve with slices of banana and whole vanilla wafers if desired.
Cover leftovers and store in the fridge for up to 3 to 4 days.
Notes
Notes:
Some cake mixes are a little smaller—13 to 14 ounces—and might require less water or oil. And certain cakes call for butter. Check the package directions to be sure of amounts, oven temperature, and times.
If garnished ahead of time, the bananas will become soggy and brown. Avoid garnishing the cake with fresh bananas until just before serving, and garnish only as many slices as needed.