Step 1

Heat the oven to 350°F (177°C) and lightly butter a 10-ounce ramekin and set it aside.

Step 2

In a small bowl, combine flour, baking soda, cinnamon, and salt together. Set aside.

Step 3

Mash the banana on a small plate with a fork.Transfer the mashed banana to a medium-sized bowl and add the sugar. With a wire whisk or an electric mixer on medium speed whip the banana and sugar together.

Step 4

Add melted butter, an egg yolk, and vanilla and mix well.

Step 5

Add the dry ingredients and mix until combined.

Step 6

Fold in the chopped walnuts.

Step 7

Pour into a lightly buttered 10-ounce ramekin and place the ramekin on a rimmed baking sheet to catch any possible spills.

Step 8

Bake for 25 to 30 minutes until golden brown.