Have the kit kat chunky, take the insulin for it and then monitor your BGs. If they go high then have a think about what to do differently next time. You may find you need to bolus earlier than normal or split the bolus. You have to be prepared to accept that they may go a little haywire until you get the hang of it.
I've been where you are now - so worried about my BGs going high and desperately trying to be 'good' (whatever that is). As others have said, your diet has to be sustainable - otherwise the condition is a lot harder to accept.
PS - I have a chocolate bar of some kind most days - it is part of my putting on weight regime following two bouts of major surgery
🙂 (I'm not suggesting you do the same

)