Preheat oven to 350°F (175°C). Line an 8×8 or 9×9 inch baking pan with parchment paper.
Mix the brownie batter according to package instructions (or homemade recipe). Pour 2/3 of the batter into the pan.
In a bowl, beat softened cream cheese until smooth. Add sugar, egg, vanilla, and salt, and mix until smooth.
Pour cream cheese mixture over the brownie batter.
Drop spoonfuls of the remaining brownie batter on top, then swirl with a knife to create a marbled effect.
Bake for 35-40 minutes, checking with a toothpick. Let cool completely before cutting. For cleaner cuts, refrigerate for at least 2 hours before slicing.