Preheat your oven to 350°F and generously grease a Bundt pan with cooking spray or butter, making sure to coat all the nooks and crannies so the cake releases cleanly. While the oven heats, mash your very ripe bananas in a small bowl until you have about 1.5 cups of mostly smooth banana puree—some small lumps are fine and actually add nice texture. Remove your eggs from the fridge to bring them to room temperature, which helps them incorporate better into the batter.

In a large mixing bowl, whisk together the yellow cake mix and instant banana pudding powder until they're evenly combined with no lumps. This step is crucial because it ensures the pudding flavor is distributed throughout the cake rather than settling to the bottom. I like to sift these dry ingredients together if you have a sifter—it aerates the mixture and helps you avoid any little pockets of unmixed pudding powder that can taste too strong.

Pour the eggs, water, oil, and vanilla extract into the bowl with the dry ingredient mixture from Step 2. Using an electric mixer on low speed, blend everything together for about 60-90 seconds until the batter is smooth and all the dry streaks are gone—be careful not to overmix, as this can make the cake tough. The batter should be thick but pourable.

Fold the mashed bananas from Step 1 into the batter using a rubber spatula with gentle strokes until they're evenly distributed throughout—I usually do about 15-20 gentle folds to keep the batter light. Pour the entire batter into your prepared Bundt pan, smoothing the top gently with your spatula. The batter should fill the pan about three-quarters full, which gives it room to rise without overflowing.

Place the Bundt pan in your preheated 350°F oven and bake for 35-45 minutes, until the top is golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs (not wet batter). The exact time depends on your oven, so start checking at 35 minutes. The cake will have a slight dome in the center and smell absolutely delicious when done.

Remove the cake from the oven and let it cool in the pan for about 15-20 minutes—this allows the structure to set so it won't break apart when you flip it. Run a thin knife or offset spatula around the edges and center hole of the Bundt pan to loosen the cake, then invert it onto a wire cooling rack or serving plate. Let it cool completely before slicing and serving, which should take another 30-45 minutes.