In a medium bowl, whisk together the almond flour, sweetener, cocoa powder, protein powder, baking powder, espresso powder, and salt.

Stir in the sour cream, egg, oil, and vanilla extract until well combined. Then add just enough liquid to thin the batter to a thick but scoopable consistency.

Set 6 silicone muffin cups into an 8-inch round cake pan. Divide the batter evenly between the muffin cups and cover the whole pan with foil, shiny side up.

Place the crisper basket in the air fryer and preheat to 300ºF. Once heated, set the cake pan on the crisper basket and choose the bake function at 300ºF.

Set the timer for 15 minutes. When the timer goes off, remove the foil from the top of the pan and return to the air fryer. Cook another 10 to 15 minutes, until risen and the tops are just firm to the touch.

Let cool completely before frosting.