Add the egg, butter, oil, milk, and pure vanilla extract in a small bowl, and mix well.
Add the dry ingredients, flour, and brown sugar to another small bowl, and mix well. Slowly fold the wet ingredients into the dry ingredients, and mix until the batter is well combined.
Add the crushed shortbread biscuit into the oven-safe mug or oven-safe ramekin, and then add the caramel, top with the butter, and add 3 pieces of chocolate.
Place the mugs or ramekins into the air fryer basket, set the temperature to 330 degrees F, and set the cooking time for 5-8 minutes.
Serve with 1 tablespoon of caramel on top, and add some whipped cream or ice cream!