Preheat oven to 350 degrees F. Line baking sheet with parchment paper. In a stand mixer, add butter, brown sugar and granulated sugar. Cream on medium speed for 3 minutes, until light and fluffy. Add egg, egg yolk and vanilla extract. Mix until well combined.
Add dry ingredients into the creamed mixture and mix until the dough just comes together. By hand, stir in chopped kit kats and mini chocolate chips. Using a cookie scoop, portion out dough 2 inches apart. Bake for 9 to 10 minutes, until lightly golden. Allow to cool for 2 minutes on the baking sheet then move to a cooling rack.