Wrap a 6-inch (15cm) cake pan with a baking pan strip or a foil strip with a damp paper towel inside. Also, line the inside of the pan with parchment paper. The easiest way to line the inside of the pan is to cut a circle out of parchment paper for the bottom and strips for the sides. Set aside.
Preheat the air fryer to 320°F (160°C).
In a large bowl, whisk together the flour, baking powder, baking soda, and salt and set aside.
In another large mixing bowl, whisk together butter and oil until combined. Then whisk in the egg to combine. Add the milk, sour cream, and vanilla and stir to combine. Stir in the sugar for about 1 minute until well mixed. Add the flour mixture and whisk just to combine.
Pour the batter into the prepared cake pan and cover with aluminum foil. Pierce a few times to allow the steam to escape while baking. Bake for 45 minutes, then remove the foil and bake uncovered for another 5 minutes if necessary. The cake is done when a toothpick inserted into the center comes out clean with a few moist crumbs.
Let cool in the pan for 15 minutes. Then remove from the pan, transfer to a cooling rack, and let cool completely. Fill and decorate to your liking or serve as is. Store in an airtight container at room temperature for up to 4 days.