1Preheat oven to 350F. Line an 8x8 pan with foil and spray with nonstick spray. Set aside.
2In a large bowl, stir together cake mix, butter and eggs until a thick batter forms. Spread HALF of the batter in prepared pan.
3In the bowl of your mixer, beat cream cheese and sugar until smooth. Scrape the sides of the bowl, then beat in egg yolk and vanilla until combined. Pour cream cheese mixture of the batter in the pan.
4Crumble remaining batter over cheesecake mixture, covering he surface of the bars as evenly as possible. Gently press with your hands to flatten, then place the pan in the oven.
5Bake for 20 minutes or until edges of bars are light golden brown.
Remove from oven and let cool completely in pan. Refrigerate until chilled, then remove foil from pan, peel from bars, and cut into squares and serve.