Gather all ingredients and ensure the sweetened condensed milk, egg yolks, and key lime juice are cold (about 30–45 minutes in the refrigerator). Cold ingredients help the filling set properly.

Pour 1 cup of chilled sweetened condensed milk into a medium mixing bowl. The milk should look smooth and creamy.

Add 4 cold egg yolks to the bowl with the condensed milk. Using a whisk or hand mixer on low speed, beat for about 1–2 minutes until the mixture is pale yellow, smooth, and well combined. Checkpoint: You are ready to move on when the mixture looks uniform with no streaks of white.

Slowly pour 1/2 cup of cold key lime juice into the bowl while stirring gently by hand with a spatula. Stir slowly for about 30–45 seconds, mixing just until the juice is incorporated. Stop as soon as the filling looks uniform; do not overmix. Overmixing can prevent the filling from setting properly in the refrigerator. Checkpoint: You are ready to move on when the filling is smooth, pale green, and there are no visible streaks.

Pour the entire filling into a prepared 9-inch graham cracker pie crust. Use a spatula to spread it evenly across the bottom and up the sides. The filling should reach close to the rim of the crust.

Sprinkle 2 teaspoons of lime zest over the top of the filling for flavor and garnish. You can use a microplane or fine grater to zest fresh limes directly over the pie.

Place the pie uncovered on a shelf in your refrigerator (not the freezer). Refrigerate for at least 2–3 hours until the filling is completely set and feels firm to the touch. If your refrigerator is very full or runs warm, allow 3–4 hours. Checkpoint: You are ready to serve when the filling no longer jiggles when you gently shake the pie.

When ready to serve, remove the pie from the refrigerator and let it sit at room temperature for 2–3 minutes for easier slicing. This makes the crust less likely to crack.

Using a sharp, thin-bladed knife, slice the pie into 6 equal pieces. Wipe the knife clean between each cut for neat slices.

Transfer each slice to a serving plate. Top with a dollop of whipped cream and a small pinch of lime zest if desired. Serve immediately.