Preheat oven to 350°F (175°C) and grease a 9-inch square baking pan with non-stick spray or parchment paper.
In a medium bowl, whisk together the gluten-free flour blend, baking soda, baking powder, salt, and ground cinnamon until well combined.
In a large mixing bowl or stand mixer, cream the softened butter, granulated sugar, and brown sugar on medium-high speed for 3-5 minutes until the mixture is light and fluffy.
Incorporate the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
Fold in the mashed bananas and sour cream until the wet mixture is homogenous.
Gradually add the dry ingredients to the wet mixture, stirring on low speed until just combined and no flour streaks remain.
Spread the batter evenly into the prepared pan and smooth the surface with an offset spatula.
Bake for 40-45 minutes, or until a wooden skewer inserted into the center of the cake emerges clean.
Transfer the pan to a wire rack and allow the cake to cool completely within the pan to set its structure before slicing or frosting.