Begin by preheating the oven to 350 degrees and preparing your bundt pan with nonstick cooking spray. Peel two bananas and mash them thoroughly in a large bowl until no large chunks remain.

Add the cake mix, pudding mix, one teaspoon of cinnamon, eggs, milk, and oil to the bowl with the mashed bananas. Using a hand mixer, beat the mixture until well combined and set aside.

Melt the butter in a small microwave-safe bowl, then stir in the brown sugar and remaining cinnamon until the mixture is thoroughly combined. Spread this butter and brown sugar mixture evenly across the bottom of your prepared cake pan.

Peel and slice the remaining two bananas, then arrange the banana slices in a single layer over the brown sugar mixture in the pan, allowing them to overlap slightly in the center for complete coverage.

Carefully pour the cake batter over the bananas in the pan, ensuring it spreads evenly without disturbing the banana arrangement. Bake for 40 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.

Remove from oven and let the cake cool in the pan for exactly 5 minutes, then carefully invert the cake onto a serving platter while still warm to prevent sticking.