Lightly grease a large microwaveable mug.
Mix together the dry ingredients until combined.
Add the egg and stir until blended.
Add the Greek yogurt and vanilla and stir well, then stir in the apples.
Pour the mixture into the mug, leaving plenty of room for rising - the cake will nearly double in height in the microwave.
Microwave on high for about 2 minutes, until risen and cooked through.
Sprinkle with ground cinnamon when cool.
Microwave cooking times vary widely so the cake needs to be watched all the time as it cooks.
Depending on your microwave it could be ready in 2 minutes or it could take up to 3 minutes.