Heat the oven to 350°F (177°C) and lightly grease a 10-ounce ramekin. In a small bowl, combine flour, baking soda, cinnamon, and salt together. Set aside. Mash the banana with a fork and add to a medium-sized bowl. Add the sugar and with an electric mixer or a wire whisk whip the banana and sugar together.
Add the melted butter, the egg yolk, and the vanilla; mix well. Add the dry ingredients and mix until combined. Pour into the ramekin and bake for 25-30 minutes until golden brown. Remove from the oven and cool slightly before eating.