Make boxed brownies.
While the brownie base is baking, make the cheesecake topping by placing the cream cheese, sugar, and salt in a large mixing bowl. Beat with a hand mixer or a stand mixer fitted with the whisk attachment on medium speed until very smooth and light, about 2 minutes.
Reduce the speed to low and add the eggs in one at a time. Scrape down the bowl, then beat in the vanilla.
Carefully pour the cheesecake topping over the par-baked brownie base. (No need to cool the brownie first.)
Dollop the reserved brownie batter over the cheesecake topping. Using a skewer or the tip of a knife, carefully swirl the batter through the cheesecake mixture.
Continue baking for 10 minutes. Reduce the oven temperature to 325°F and bake for another 20 to 25 minutes or until the cheesecake topping is puffed around the edges and the center wobbles slightly.
Place the pan on a wire rack to cool completely. Refrigerate for 1 hour before slicing.
