Preheat your oven to 350°F and grease an 8-inch square baking pan with nonstick spray or a light coat of oil.
In a medium mixing bowl, whisk together the flour and baking soda until fully combined.
In a separate large mixing bowl, combine the brown sugar, water, mashed banana, canola oil, and vanilla extract. Stir until smooth.
Gradually add the dry ingredients to the wet ingredients, stirring just until the batter is moistened. Be careful not to overmix.
Pour the batter into the prepared baking pan and smooth the surface evenly.
Bake for 30–35 minutes, or until a toothpick inserted into the center of the cake comes out clean and the top is golden brown.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Once cooled, dust the top with confectioners' sugar. Slice into 9 pieces and enjoy!