1 ripe banana, 1/4 cup coconut oil, 1/2 cup brown sugar, 1 cup all-purpose flour, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, 1/4 teaspoon salt, 1/2 teaspoon cinnamon, 1/4 cup plant-based milk, 1 teaspoon vanilla extract, 1/4 cup chopped walnuts (optional)