Preheat your oven to 350°F (175°C). This ensures that your cake bakes evenly.
Grease your 9x13 inch baking pan with butter or non-stick spray, and set aside. This will prevent the cake from sticking.
In a large mixing bowl, beat the softened butter and granulated sugar together with an electric mixer until the mixture is light and fluffy, about 2-3 minutes.
Add the eggs, one at a time, mixing well after each addition. Then, add the sour cream and vanilla extract, and mix until combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. This ensures that the leavening agent is evenly distributed.
Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the mashed bananas with a rubber spatula, ensuring they are evenly incorporated.
Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely before frosting.
In a mixing bowl, beat the softened cream cheese until smooth. Add the milk and vanilla extract, mixing until well combined. Gradually add the sifted powdered sugar, beating until the frosting is smooth and creamy. If needed, add an extra tablespoon of milk to reach your desired consistency.
Once the cake is completely cool, spread the cream cheese frosting evenly over the top. You can also add some chopped nuts or banana slices on top for extra flair!