Add Vinegar to your Toffee

I made toffee today :) Definitely a highlight of the week, but it's very moreish and I know it's just going to make me feel ill. It'll be good when I get another sugar craving. 

When we found the recipe on the internet it said to add vinegar to in which we all (me, my sister and my brother) disbelieved instantly. Turns out it makes it go hard or something. So I think I need to be more trusting - generally, not just with strangers on the internet.