Prep the pan (5 minutes): Preheat your oven to 325°F and position the rack in the lower third of the oven. Get a 9-inch springform pan and set it aside. You'll need to wrap it later, so have heavy-duty foil ready.

Make the crust (10 minutes, then 8 minutes bake): Combine 2 cups graham cracker crumbs, 0.5 cup granulated sugar, and 6 tablespoons melted unsalted butter in a bowl. Mix with a fork until the mixture looks like wet sand with no dry crumbs. Press this mixture firmly into the bottom of your springform pan and slightly up the sides, using the bottom of a measuring cup to pack it tightly. Bake for 8 minutes until the crust is lightly golden and fragrant. Checkpoint: The crust should smell toasted and feel firm when you press it. Remove from oven and let cool for 5 minutes.

Bring cream cheese to room temperature (30 minutes if needed, or 5 minutes if starting with room temp): If your cream cheese is cold, let it sit on the counter. Cold cream cheese will not mix properly and create a lumpy filling. Checkpoint: The cream cheese should feel soft to the touch, not firm or cold.

Blend the cream cheese (3 minutes at low speed): Using an electric mixer on LOW speed, beat 32 ounces (four 8-oz packages) of room-temperature cream cheese for about 2 minutes. Stop the mixer and scrape down the sides and bottom of the bowl with a rubber spatula. This removes any unblended chunks. Checkpoint: Your cream cheese should look smooth and creamy with no visible lumps.

Add sugar (2 minutes at low speed): Slowly sprinkle 1 cup granulated sugar into the mixer while it runs on low speed. Beat for about 1 minute, then scrape down the bowl again. The mixture should look pale and fluffy. Checkpoint: No grits or sugar crystals should be visible between your fingers when you rub the mixture.

Add sweetened condensed milk (1 minute at low speed): Pour in 0.75 cup sweetened condensed milk and mix on low speed until fully blended. Scrape the bowl again. The filling will start to look smoother and more spreadable.

Combine lime juice and zest separately (2 minutes): In a separate mixing bowl, whisk together 0.5 cup fresh key lime juice (about 12–15 key limes, or bottled key lime juice), 2 tablespoons of key lime zest (the colored part of the lime skin only, not the white pith), 3 large room-temperature eggs, 1 tablespoon cornstarch, 0.5 teaspoon vanilla extract, and 0.25 teaspoon salt. Whisk by hand until the eggs are fully mixed and no streaks of egg white remain visible. The cornstarch will help thicken the filling. Checkpoint: The lime mixture should smell bright and zesty, and look uniform in color with no visible chunks of egg white.

Gently combine the two mixtures (2 minutes at low speed): Pour the lime mixture into the cream cheese mixture SLOWLY while the mixer runs on low speed. As soon as the ingredients are no longer visibly separate, STOP THE MIXER. Do not overmix, as this can add too much air and cause cracks. Scrape down the bowl one final time. Checkpoint: The filling should be pale green, smooth, and pourable but not thin like milk.

Prepare the water bath setup (5 minutes): Tear off a large sheet of heavy-duty aluminum foil (about 18 inches long). Wrap it tightly around the outside and bottom of your cooled springform pan, making sure there are no gaps or holes where water could seep in. You want the pan to be completely sealed. Place the wrapped pan inside a larger roasting pan (at least 2 inches taller than the springform).

Pour filling and set up water bath (3 minutes): Pour the green filling mixture into the crust-lined springform pan and smooth the top gently with a spatula. Place the roasting pan (with the springform inside) on the oven rack in the lower third. Pour hot water from a kettle into the roasting pan until it comes halfway up the outside of the springform pan (about 1–1.5 inches of water). The water bath keeps the cheesecake moist and prevents cracks. Checkpoint: You should not see any water inside the springform pan, only around it.

Bake the cheesecake (45–50 minutes at 325°F): Close the oven door gently and set a timer for 45 minutes. After 45 minutes, check the cheesecake by looking at the edges and gently shaking the pan. The edges should be set and no longer jiggly, but the very center (about a 2-inch circle in the middle) should still jiggle slightly like soft Jell-O when you gently move the pan. Checkpoint: If the center is completely firm, it's overbaked and may crack. If it's too jiggly (more than 3 inches of movement), bake for 2–3 more minutes. If your oven runs hot, it may finish closer to 45 minutes; if it runs cool, it might need 50 minutes.

Make sour cream topping while cheesecake finishes (2 minutes): While the cheesecake is in the oven during the last 5 minutes, whisk together 1 cup room-temperature sour cream, 2 tablespoons granulated sugar, and 0.5 teaspoon vanilla extract in a small bowl. Whisk until smooth and completely combined with no lumps. Checkpoint: The topping should be thick but spreadable, like yogurt, with no sugar grains visible.

Add sour cream topping (3 minutes): Remove the cheesecake from the oven but leave it in the hot water bath. Using a spatula, spread the sour cream topping evenly over the surface of the warm cheesecake, starting from the center and working outward. You don't need to cover the very edges completely. Checkpoint: The topping should be smooth and white, covering about 80% of the top surface.

Bake with topping (8 minutes at 325°F): Return the cheesecake to the oven for exactly 8 minutes. The topping should look set and matte (not shiny) when you remove it. Do not overbake the topping, or it may curdle or brown slightly. Checkpoint: The topping should not jiggle when you gently shake the pan, and it should look smooth and matte.

Cool in water bath (30 minutes): Remove the cheesecake from the oven and leave it sitting in the hot water bath for 30 minutes. This gradual cooling prevents cracks from forming as the cheesecake contracts. Do not remove the cheesecake from the water bath yet.

Cool at room temperature (1 hour): After 30 minutes, carefully remove the springform pan from the roasting pan and set it on the counter. Cool the cheesecake at room temperature for about 1 hour. The residual heat will continue to set the filling. Checkpoint: The cheesecake should feel cool to the touch but not cold.

Chill completely (6 hours or overnight): Once the cheesecake has cooled to room temperature, place it in the refrigerator for at least 6 hours, or preferably overnight. Chilling allows the flavors to develop and the texture to become perfectly creamy. The cheesecake will taste much better cold. Checkpoint: The cheesecake should be completely cold and firm before slicing.

Remove from springform pan and slice (5 minutes): When ready to serve, run a thin, hot knife around the inside edge of the springform pan, wiping the knife clean between each pass to prevent dragging. Unlatch and remove the springform collar carefully. Using a clean, hot knife (dipped in hot water and wiped dry after each cut), slice the cheesecake into 8–12 pieces, depending on your preferred serving size. The knife should glide smoothly through the creamy filling. Checkpoint: Each slice should have clean edges and a uniform pale green color with no cracks.

Garnish and serve (2 minutes): Transfer each slice to a serving plate using a thin spatula or cake server. Top with a dollop of freshly whipped cream, a thin slice or wheel of fresh lime, and a small sprinkle of graham cracker crumbs for texture. Serve immediately. Checkpoint: The cheesecake should slice cleanly without crumbling, and the whipped cream should sit proudly on top.