In a mixing bowl, combine the cake mix with the eggs and melted butter. Mix until smooth and then pour into a greased 9X13 pan.
In a separate bowl, beat the cream cheese with the sugar and vanilla until smooth. Place tablespoon sized dollops of the cream cheese filling evenly spread on top of the cake batter in the pan.
Use a knife or skewer to swirl the cream cheese filling into the cake batter. The cake batter is pretty thick because it is more like cookie dough, so it takes a little bit of effort to swirl the cream cheese filling in. Don't worry if it doesn't seem to be very pretty - it will all be fine once it bakes! If desired, add some sprinkles on top.
Bake for 20-23 minutes or until toothpick inserted in the center comes out clean. The edges should be a light golden brown. Let the bars cool completely and then slice and serve.