Preheat the air fryer to 310°F and line a 6-inch round cake pan with parchment paper and set aside.
In a large bowl, whisk together the flour, cinnamon, salt, nutmeg, and baking soda.
In a separate bowl, mix together the mashed banana, beaten eggs, sugar, sour cream, vegetable oil, and vanilla until combined.
Gently stir the wet ingredients into the dry ingredients until combined.
Pour the batter into the prepared cake pan and top with walnuts.
Cook at 310°F for 15 minutes, rotate the pan 180°, then cook for an additional 15 to 20 minutes until a toothpick or cake tester inserted into the center of the bread comes out clean.
Let the cake cool in the pan for an hour before slicing.