Chcocloate mug cake

recipe url: 
https://www.bbcgoodfood.com/recipes/microwave-mug-cake

4 tbsp self-raising flour
4 tbsp caster sugar
2 tbsp cocoa powder
1 medium egg
3 tbsp milk
3 tbsp vegetable oil or sunflower oil
a few drops of vanilla essence or other essence (orange or peppermint work well)
2 tbsp chocolate chips, nuts, or raisins etc (optional)