Step 1: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or nonstick spray.
Step 2: In a large bowl, prepare the yellow cake mix according to package directions, typically combining the cake mix with 3 large eggs, ½ cup vegetable oil, and 1 cup water. Use a hand mixer on medium speed for 2 minutes, scraping the bowl sides once. Pour the batter into the prepared pan, smoothing the top.
Step 3: Bake for 22–25 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 10 minutes.
Step 4: While the cake is still warm, use the handle of a wooden spoon to poke 30–40 evenly spaced holes across the cake surface.
Step 5: Slowly pour the entire can of sweetened condensed milk over the cake, ensuring it seeps into all the holes.
Step 6: In a separate medium bowl, whisk together the instant banana pudding mix and 2 cups cold milk for about 2 minutes, until the mixture thickens. Gently fold in the 2 sliced ripe bananas. Spread the pudding mixture evenly over the cake.
Step 7: Cover the cake with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld and the pudding layer to set.
Step 8: For the whipped topping, chill a large mixing bowl and beaters in the freezer for 10 minutes. Combine 2 cups heavy whipping cream, ¼ cup powdered sugar, and 1 teaspoon vanilla extract in the chilled bowl.
Step 9: Beat on medium-high speed until stiff peaks form.
Step 10: Remove the cake from the refrigerator and gently spread the whipped topping evenly over the pudding layer. Sprinkle the 1 cup of crushed vanilla sandwich cookies evenly on top. Arrange extra banana slices for garnish, if desired.
Step 11: Return the cake to the refrigerator for an additional 30 minutes before serving to allow the whipped layer to firm up.