Place egg, milk and maple syrup in a small bowl and lightly whisk together until combined.
Stir in the chocolate mug cake mix and a pinch of salt until a smooth batter forms.
Pour batter into a large mug or small bowl, and microwave for 70-90 seconds (depending on mug size) until puffy on top but slightly gooey. Once cooked, serve immediately - top with yoghurt and berries if you like!