Don't stress. At least it wasn't a kilogram bar of dairy milk or some such.
I understand the feeling guilty, but just try to get past that. You are in control here. It's YOUR diet and what you put into your body is YOUR choice. So today you chose to have an extra bar. That's your prerogative. Tomorrow you'll most likely choose not to, and that's cool, too.
Recognise these choices, and you'll see that the overwhelming majority of the time you'll choose to follow the plan to the letter.
Food should never make us feel guilty - if it does, you're letting it be the boss. You're the boss!