Combine the mashed banana, kefir, extra-virgin olive oil, honey, egg, and vanilla extract in a large bowl. Whisk until smooth.
Add the whole wheat flour, all-purpose flour, baking powder, baking soda, salt, and cinnamon. Fold with a spatula until just combined; some lumps are ok. Fold in the walnuts.
Fill individual muffin cups with batter.
Insert the crisper plates into both COSORI Air Fryer baskets.
Select Zone 1, select the Bake function, adjust the temperature to 150°C and time to 5 minutes, select Match, then tap Start/Pause to preheat.
Insert the muffins onto the preheated crisper plates.
Select Zone 1, select the Bake function, adjust the temperature to 150°C and time to 16 minutes, select Match, then tap Start/Pause.
Remove muffins when golden brown and a toothpick inserted into the center comes out mostly clean, a few moist crumbs are ok.
Allow muffins to cool for 10-15 minutes before serving.
