Melt the chocolate, condensed milk and vanilla together in a large saucepan.
Once melted, spread half of the mixture in an even layer in the prepared pan. Top with fingers of KitKats. You should use around 280-300g of the KitKats.
Top with the remaining mixture and crumble up the remaining KitKats to decorate on top.
Place in the fridge to set.
Mine was firm enough to slice into pieces in one hour but you could let it set it in four hours to have a solid fudge.