In a medium bowl, whisk egg until well beaten. Add peanut butter, brown sugar, vanilla, baking soda and salt. Stir until well combined and mixture thickens a bit, about 15 seconds. Fold in the chocolate chips.
Roll dough into balls. Flatten each ball to a disk about 4cm wide. Transfer to a plate.
Line an air fryer basket with foil. Working in batches, arrange cookies in basket, spacing little bit apart. Cook at 180 degrees until deeply browned, 5-7 minutes.
Carefully, remove foil with cookies from air fryer basket. Transfer to a wire rack and let cool on foil until cookies firm up, about 5 minutes. Using a thins spatula, transfer cookies to a wire rack and let cool completely.
Whisk together the egg, yoghurt, milk, oil and vanilla. Stir through the caster sugar and mix until combined.
Sift in the flour and cocoa powder, then add the salt and most of the chocolate chips. Stir until just combined, with no floury streaks.
Arrange the cake cases and scoop batter evenly into each case. Sprinkle the reserved chocolate chips on top.
When preheated, remove the basket from the airfryer and put the cake cases into it. Arrange them close together, just touching.
Bake for 12-15 minutes until well-risen and the tops of the muffins spring back when pressed. Lift out of the airfryer and allow to cool on a wire rack for 15 minutes before serving.
