Preheat oven to 350-f degrees.
For the crust, mix together the dry cake mix, butter, and 1 egg. Mix well. The mixture will be thick. Press into a greased 9×13 baking pan and set aside.
For the filling, beat cream cheese until smooth. Add the vanilla extract, eggs, and melted butter. (If you are using any zest, add it now) Mix together well and add powdered sugar. Mix until the powdered sugar is fully incorporated.
Spread filling over crust and bake for 38-42 minutes, until lightly brown and when gently shaken the center will only slightly jiggle.
Cool and cut into small bars. (I let my bars chill in the fridge for a bit before cutting) Cover and store leftovers in the fridge.