Preheat your oven to 350°F (175°C) and grease your 9x13" baking pan.
Prepare the brownie mix according to the package instructions and bake for about 25-30 minutes.
Combine the white cake mix, eggs, water, and oil in another bowl and beat until smooth.
Beat the cream cheese and butter until smooth, then mix in powdered sugar, vanilla, and sweetened condensed milk.
Pour the cake batter over cooled brownies and dollop cream cheese mixture on top, swirling it gently.
Bake the assembled cake for an additional 30-35 minutes until a toothpick comes out clean.