Prep: Preheat your oven to 350°F. Spray a 9×13-inch baking pan with cooking spray then line it with parchment paper, but with the parchment paper hanging off the sides to make it easy to lift out bars.: The room will fill with warmth and a faint lemon perfume as the oven comes to temperature, and you will feel ready when the pan is lined properly. Start by preheating your oven to 350°F , then prepare a 9×13 inch baking pan by spraying it lightly and lining it with parchment paper that overhangs the sides for easy removal. When the oven hums and the pan is ready, you gain both confidence and cleanliness in the kitchen. The parchment makes lifting the bars simple, and if you skip this step the crust can stick to the pan which makes slicing messy. A common mistake here is not leaving enough overhang, so be generous with the paper to avoid fussy extraction.
Make Crust: Add all the crust ingredients to a food processor and pulse a few times until mixture resembles a dry coarse meal.: As you pulse the dry ingredients, a light, sandy texture will form under the blades, smelling faintly of butter and lemon oil. Place 1 cup all purpose flour , 1/4 cup cornstarch , 1/4 cup granulated sugar , 1/2 cup butter , 1/2 teaspoons salt , and 2 tablespoon lemon zest into a food processor and pulse until the mixture resembles a dry coarse meal. The aroma of lemon zest brightens the crumb and the butter gives it that shortbread richness. If you over-process, the mixture will become paste like, so pulse just until combined. A frequent error is adding too much moisture, which prevents the crust from firming properly during bake.
Bake Crust: Transfer the crumb mixture to the prepared baking dish and press it down evenly into the bottom. Transfer the baking dish to the oven and bake for 15 minutes.: When you press the crumb into the pan, the tactile feedback will tell you if it is packed correctly, and you want an even, compact base that holds together. Transfer the crumb mixture to the prepared baking dish and press it down evenly into the bottom, applying steady pressure to create a uniform layer. Slide the pan into the oven and bake for 15 minutes until the surface looks set and the edges take on a light golden tint. The smell of warm butter will become more pronounced as the crust browns. If you underbake it, the bottom can be soft and difficult to slice, so ensure it is firm. Avoid pressing so hard that the crust becomes overly dense, which can detract from its tender bite.
Make Cheesecake Layer: Using a hand mixer, mix the softened cream cheese until smooth. Add the eggs, sugar, sour cream, vanilla and salt, and continue to mix until the batter is well combined and smooth.: You will notice a silky sheen form as the cream cheese becomes smooth under your mixer, and that is the texture you want. Using a hand mixer, blend the softened 24 ounce cream cheese until smooth and lump free. Add 3 large eggs , 1 cup granulated sugar , 1/2 cup sour cream , 2 teaspoon vanilla extract , and 1/4 teaspoon salt , and continue mixing until the batter is well combined and velvety. The batter should be smooth and pourable, not airy. Over mixing after adding the eggs traps air which can cause cracks during baking, so mix just to incorporate. If you end up with small lumps, scrape the bowl and beat gently until smooth.
Bake The Cheesecake: Pour the cheesecake batter over the baked crust and bake for 45 minutes or until the top of the cheesecake has browned slightly. Set the cheesecake aside until it fully cools down.: As the oven warms the batter, it will set from the edges inward, and you are aiming for a gentle wobble in the center when it is done. Pour the cheesecake batter over the pre baked crust and smooth the surface. Return the pan to the oven and bake for 45 minutes or until the top has browned slightly and the center no longer looks liquid, though it may still have a small tremble. The top should be pleasantly golden but not deeply cracked. Let the pan sit to cool down; rushing this step can cause the filling to crack or collapse. A common oversight is opening the oven door during baking, which can lead to temperature loss and affect the set.
Add Lemon Curd: Pour the lemon curd over the cooled cheesecake and spread it evenly with a spatula. Refrigerate for at least 3 hours, or until the cheesecake and lemon curd layers set.: After the cheesecake has cooled and the surface is calm to the touch, the lemon curd will drape across it like a glossy blanket, offering a vivid citrus note. Pour 1 1/2 cups lemon curd over the cooled cheesecake and spread it evenly with a spatula so the top is smooth and shiny. Refrigerate the assembled bars for at least 3 hours so both layers firm and integrate. If you try to cut into the bars before they are fully chilled, the layers can smear and the slices will not hold their shape. Also, spreading curd on warm cheesecake can cause it to sink into the filling rather than sitting as a distinct layer.
Cut & Serve: Carefully lift the cheesecake out of the pan and slice it into your desired size bars. I cut it 4 x 6, making 24 bars.: There is a satisfying click when your knife meets a clean, chilled edge, and that tells you the bars are ready to be portioned. Carefully lift the cheesecake from the pan using the parchment overhang and place it on a cutting board. Slice into bars in your desired size, noting that cutting a 4 by 6 grid yields 24 bars as I do. For neat slices, use a hot, sharp knife wiped between cuts. If you try to slice while the bars are still warm or without wiping the knife, you will get ragged edges and sticky crumbs, so chill thoroughly and clean the blade often.