1. Preheat the air-fryer to 160°C.
2. Whisk together the egg, yogurt, milk, oil and vanilla. Stir through the caster sugar and mix until combined.
3. 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.
4. Arrange the cake cases on the work surface and carefully spoon the batter among them evenly. Sprinkle the reserved chocolate chips on top.
5. When preheated, remove the basket from the air-fryer and carefully lift the filled cake cases into it. Arrange them close together, just touching if possible.
6. Bake for 12-15 mins until well-risen and the tops of the muffins spring back when pressed. Lift out of the air-fryer and allow to cool on a wire rack for 15 mins before serving.
Tip: Check how many cake cases fit into your air-fryer before using it – you may need to make the muffins in 2 batches.
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