Preheat Air Fryer by setting it to 350 degrees for 4 minutes.
In a mixing bowl, whisk together flour, cornstarch, sugar, baking powder, salt, and nutmeg.
In a separate mixing bowl, whisk together buttermilk*, melted butter, 2 eggs, and egg yolk.
Add wet ingredients to dry ingredients and mix until just combined. Do not over mix!
Add 8 silicone muffin cups to the Air Fryer basket. Using an ice cream scoop and 1/2 of the batter, fill muffin cups 1/2 of the way full.
Set the Air Fryer to 340 degrees and bake for 12-14 minutes until tops are golden brown and a toothpick inserted in the center comes out clean.**
Remove donuts from muffin cups, and allow to cool on a baking rack for 5 minutes.
Repeat cooking process for remaining 1/2 of the batter.
**Oven Cooking Instructions: Spray a 12-cup muffin tin with cooking spray. Portion the batter evenly into each muffin tin cavity. In a preheated 400 degree oven, bake the donut muffins for 18-22 minutes, rotating the pan halfway through cooking.