Preheat your oven to 175°C (350°F) and grease a 23 x 33 cm (9 x 13-inch) baking pan, lining the bottom with parchment paper.
Whisk the flour, baking soda, baking powder, salt, and cinnamon together in a medium bowl.
In a separate large bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
Stir in the mashed bananas.
Alternately add the dry ingredient mixture and the sour cream/milk mixture to the wet ingredients, starting and ending with dry, mixing until just combined.
Pour the batter into the prepared pan and smooth the top.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Cool completely in the pan before serving or frosting.