Preheat the oven to 325°F (163°C). Butter a 9-inch springform pan and line the bottom with parchment.

Combine graham cracker crumbs and melted butter in a bowl. Press evenly into the pan bottom. Chill for 10 minutes, then bake for 8–10 minutes until set. Cool slightly.

Beat the softened cream cheese until smooth and lump-free. Scrape the bowl.

Add sugar, cornstarch, and salt. Beat until incorporated.

Mix in sour cream, lemon juice, lemon zest (from 2), and vanilla. Keep the mixer low to avoid adding air.

Add eggs one at a time, mixing gently until just combined. Overbeating introduces air and can cause cracks.

Pour filling into the crust and smooth the top. Place the springform pan in a shallow roasting pan. Optional: pour hot water into the roasting pan halfway up the springform sides for a water bath—this reduces cracking.

Bake for 50–65 minutes. The edges should be set; the center will still have a slight wobble.

Turn off the oven, crack the door, and let the cheesecake sit inside for 30 minutes to cool gently. Remove and cool to room temperature. Chill for at least 4 hours, preferably overnight.

In a medium saucepan, combine lemon juice, lemon zest, and sugar. Warm over low heat until sugar dissolves.

In a separate bowl, whisk whole eggs and extra yolks until combined. Temper the eggs by slowly whisking a ladle of warm lemon mixture into the eggs. Then whisk the egg mixture back into the saucepan.

Cook over low-medium heat, stirring constantly, until the curd thickens and reaches about 170–175°F (77–80°C). It should coat the back of a spoon. Do not boil.

Remove from heat and whisk in butter pieces until fully incorporated and glossy. Season with a pinch of salt. Strain through a fine sieve if you want an ultra-smooth curd. Cool to room temperature, then chill.

When cheesecake is fully chilled, spread a thin layer of curd over the top or spoon and gently swirl for a marbled effect. Refrigerate until the curd is set.

Run a thin knife around the springform edge before releasing to get clean sides. Slice with a hot, dry knife for neat pieces.