Preheat your oven to 325°F (160°C). Line a 9x9 inch baking pan with parchment paper, leaving some overhang for easy removal.

In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Stir until the mixture resembles wet sand.

Firmly press the mixture into the bottom of the prepared baking pan. Bake for 10 minutes, then remove from the oven and let cool.

In a large mixing bowl, beat the softened cream cheese with an electric mixer until smooth and creamy.

Gradually add the sugar, mixing until well combined. Add the eggs one at a time, mixing well after each addition.

Mix in the vanilla extract, sour cream, lemon juice, and lemon zest until smooth and creamy.

In a small saucepan over medium heat, combine the sugar, lemon juice, cornstarch, and lemon zest. Stir until the mixture thickens slightly, about 2-3 minutes. Remove from heat and let cool.

Pour the cheesecake filling over the cooled crust, spreading it evenly.

Drop spoonfuls of the lemon swirl mixture over the cheesecake filling. Use a knife or toothpick to gently swirl the lemon mixture into the cheesecake for a marbled effect.

Bake in the preheated oven for 30-35 minutes, or until the center is set but still slightly jiggly. Turn off the oven and leave the cheesecake bars inside for an additional hour.

Remove the cheesecake bars from the oven and let them cool to room temperature. Once cooled, refrigerate for at least 4 hours, or overnight for best results.

Using the parchment overhang, lift the cheesecake out of the pan. Slice into bars and serve chilled.