Preheat your oven to 350°F (175°C). Lightly grease and flour a 9x13 inch baking pan, or spray generously with non-stick baking spray. Set aside.

In a large mixing bowl, combine the yellow cake mix, 1/2 cup melted butter, 1 large egg, and 1/2 mashed banana. Mix with a sturdy rubber spatula or an electric mixer on low speed until just combined and a thick, uniform dough forms. Do not overmix.

Press the dough evenly into the bottom of the prepared 9x13 inch baking pan, creating a compact crust. You can use your hands or the back of a spoon for this step.

If not already done, cook the 4 slices of bacon until very crispy. Drain them thoroughly on paper towels to remove excess grease. Once cooled, crumble the bacon into small, bite-sized pieces. Set aside.

In a separate large mixing bowl, using an electric mixer, beat the softened cream cheese and creamy peanut butter together on medium speed until the mixture is smooth, light, and creamy, about 2-3 minutes. Scrape down the sides of the bowl as needed.

Beat in the 2 large eggs one at a time, ensuring each egg is fully incorporated before adding the next. Stir in the vanilla extract.

Gradually beat in the 1/2 cup melted butter (ensure it's cooled slightly, not hot) until fully incorporated and the mixture is smooth.

With the mixer on low speed, slowly add the powdered sugar, mixing until just combined and the topping is smooth. Do not overmix once the sugar is added.

Gently fold in the 1 mashed banana with a spatula until just combined. The banana adds to the 'gooey' texture and flavor.

Carefully pour the gooey topping mixture evenly over the cake base in the prepared pan. Spread it gently with a spatula to cover the entire crust.

Place the pan in the preheated oven and bake for 45-55 minutes. The cake is done when the edges are lightly golden and set, but the center still jiggles slightly when the pan is gently shaken. A wooden skewer inserted into the gooey center should come out with moist crumbs, not wet batter.

Remove the cake from the oven. Immediately sprinkle the crumbled crispy bacon evenly over the warm cake. The warmth will help the bacon adhere.

Let the cake cool completely on a wire rack before cutting and serving. This is crucial for the gooey center to set properly and achieve its characteristic texture. Cutting it warm will result in a very messy, though still delicious, experience.