Preheat oven to 350 degrees. Light grease or spary with nonstick spray, a 9x13" baking pan.
Using a mixer, in a large bowl, add cake mix, egg, and butter and mix well.
Pat cake mix mixture into the bottom of prepared pan and set aside.
Using mixer and bowl, beat cream cheese until smooth, then add in the eggs and vanilla.
Add powdered sugar and beat very well, then reduce the speed of mixer and gradually add in melted butter and incorporate well.
Stir in the chocolate chips.
Pour cream cheese filling onto cake mixture and spread evenly.
Bake for 40-50 minutes.
You want to center to be a little gooey, so don't overbake.
Remove from oven and scatter the milk and semi-sweet chocolate chips on top of the bars and let sit several minutes to let them soften and get all melty. Then using a knife, spread the chocolate around covering the entire pan of bars.
Allow to cool for at least another hour and then cut into bars.