Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.
In a large bowl, mix the yellow cake mix, melted butter, and one egg until a thick dough forms. Press this mixture evenly into the bottom of the prepared baking dish.
In a separate bowl, beat the softened cream cheese until smooth. Add in the eggs and vanilla, and mix until fully combined. Gradually beat in the powdered sugar until smooth and creamy. If using, fold in white chocolate chips or nuts.
Pour the cream cheese mixture over the cake base and spread it evenly. Bake in the preheated oven for 35–40 minutes. The center should still jiggle slightly—don’t overbake! It will firm up as it cools.
Let the bars cool completely in the pan, then refrigerate for at least 1 hour before slicing. Cut into squares and enjoy!