01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper or lightly grease with butter.
02 - Combine cake mix, melted butter, and egg in a mixing bowl. Mix until soft dough forms. Press evenly into bottom of 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 until fluffy and smooth.
04 - Pour lemon mixture over base and spread evenly. Bake 35-40 minutes until edges are set and center is slightly jiggly. Avoid overbaking to maintain gooey texture.
05 - Let cake cool completely before cutting into squares. Dust with additional powdered sugar if desired. Serve with fresh berries or whipped cream.