For the Lemon Cheesecake Bars:

1. Remove all cheesecake ingredients from the fridge 1 hour before baking to come to room temperature.
2. Start by making your lemon curd. Add sugar, lemon zest, and cornstarch to a cold small saucepan and mix to combine. Whisk in the egg until smooth, then whisk in the lemon juice. Set the saucepan to low heat and cook, whisking constantly until the mixture starts to thicken. Continue to cook on low until the lemon curd thickens enough to coat the back of a spoon. Remove from the heat and whisk in the butter until emulsified and smooth. Strain into a jar to remove the lemon zest and place in the fridge to cool fully before using. It will thicken up as it chills. If making the curd ahead of time, remove the curd from the fridge with the rest of the cheesecake ingredients to come to room temperature. 
3. Preheat the oven to 350°F. 
4. Pulse the graham crackers in a food processor until you have fine crumbs. Add the sugar, lemon zest, and melted butter and pulse until the mixture resembles wet sand. If using unsalted butter, add ¼ – ½ tsp salt. 
5. Grease a 9×13” baking dish with butter and line with parchment paper with excess overhang to easily remove the cheesecake after it cools. Add graham cracker crumbs to the baking dish and press into the pan using the bottom of a measuring cup. Bake for 10 minutes to set the crust, then remove and let cool fully. 
6. Reduce the oven temperature to 300°F. 
7. In a large bowl, add the softened cream cheese and sugar and blend using a stand mixer or electric mixer until fluffy and combined. Add the sour cream, eggs, egg yolk, vanilla, lemon zest, lemon juice, and salt. Mix until combined. 
8. Once the graham crust has cooled to touch, add the cheesecake mixture to the pan and spread out evenly to all sides. Add spoonfuls of the lemon curd about 1-2” spaced apart, then using a chopstick or toothpick, swirl the lemon mixture into the cheesecake. Avoid overmixing so you have distinct swirls. 
9. Place a large pan filled with hot water on the bottom rack. Place the cheesecake bars in the oven on the middle rack above it. This will help create steam to cook your cheesecake more evenly and help prevent cracks. 
10. Bake the cheesecake at 300°F for 60-65 minutes or until the outside 3” borders are set but the middle is still jiggly. It will continue to set as it cools. 
11. Remove from the oven and let it sit at room temperature for 2-3 hours, then transfer to the fridge for 4 hours or overnight. 
12. When ready to serve, use the parchment paper overhang to carefully lift the cheesecake out of the pan. Cut into squares, wiping the knife in between slices for clean cuts. To store, keep in an airtight container in the fridge for up to 4-5 days.