01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper or lightly grease it.
02 - In a mixing bowl, combine the cake mix, melted butter, and egg. Mix until a soft dough forms.
03 - Press the dough evenly into the bottom of the prepared pan to form the base.
04 - In another bowl, beat cream cheese until smooth. Add eggs one at a time, then mix in powdered sugar, lemon juice, lemon zest, and vanilla extract until fluffy and smooth.
05 - Pour the lemon mixture over the base and spread evenly. Bake for 35–40 minutes, until the edges are set and the center is slightly jiggly (do not overbake).
06 - Allow cake to cool completely before cutting into squares. Dust with extra powdered sugar if desired.