Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9-inch square baking dish.
Beat superfine sugar and butter in a bowl with an electric mixer until smooth. Add eggs one at a time, beating until each egg is fully incorporated before adding the next. Mix baking soda into sour cream in another bowl and stir into butter mixture until combined.
Stir bananas into butter mixture; add cake flour and baking powder and mix well. Stir in vanilla extract. Pour banana cake batter into prepared baking dish.
Mix pecans, brown sugar, and cinnamon in a bowl; sprinkle over cake batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes.