Mix the cake mix, egg and 8 T. of the melted butter with an electric mixer. Pat into the bottom of a 13x9-inch pan that has been sprayed with cooking spray and lined with parchment paper.
With an electric mixer beat the cream cheese until smooth. Add the eggs, vanilla and the rest of the melted butter. Mix well. Add the powdered sugar and mix until smooth. Spread this over the cake batter and bake for 40-50 minutes. Do not overbake – the cake center should still be gooey!