Preheat oven to 350 degrees and line cookie sheets with parchment paper.
Beat the sugar and butter together until nice and creamy, then beat in eggs and extra egg yolk, one at a time. Add in salt, baking soda and flour and beat lightly until thoroughly combined. Stir in the chocolate chips and Kit Kat bars - don't beat them in, just stir with a spoon so that they don't get broken up.
Refrigerate the dough for about two hours.
Scoop the dough by heaping tablespoons and roll into balls, placing about two inches apart on the cookie sheet. Bake for 8 minutes for tender and chewy cookies, or up to 10 minutes for crispier cookies.
Allow to cook for about ten minutes before devouring!
This recipe makes about two dozen cookies, depending on how big you make them!