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