Preheat oven to 350 degrees F. Line a 9 by 13" rectangular baking dish with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.
In the bowl of a stand mixer, combine the cake mix, melted butter, and egg with the paddle attachment and beat slowly until a dough comes together. Press the dough evenly into the bottom of the prepared pan.
Take pieces of the cookie dough and flatten them in the palm of your hand. Gently press the pieces of cookie dough together on top of the crust layer, like you're putting together a puzzle, until the cookie dough layer is even. Set aside.
Wipe out the stand mixer bowl and add in the cream cheese. Beat until smooth and soft, 1 minute. Gradually add in the eggs, one at a time, beating well after each addition. Beat in the confectioners' sugar slowly until combined, followed by the vanilla. Lastly, slowly pour in the melted butter with the mixer on low speed until incorporated. Pour the batter evenly over the layers in the pan and smooth out the top.
Bake for 35-45 minutes or until the center is just about set. The center may jiggle slightly, but it should not be sloshing around in the pan. You do NOT want to over-bake these; they're called gooey bars for a reason and they'll continue to set up as they cool. Cool completely, then refrigerate for at least 2 hours, up to overnight, to set before cutting into bars. Just before serving, garnish with additional confectioners' sugar and miniature chocolate chips, if desired.