Mix graham cracker crumbs and sugar in a bowl. Add melted butter and stir until evenly coated. Press mixture into a 9-inch pie dish.
1 ½ cups graham cracker crumbs, ⅓ cup granulated sugar, 7 tablespoons unsalted butter
Bake for 8-10 minutes. Let cool.
In a large bowl or a stand mixer with a whisk attachment, whisk egg yolks until pale and thickened. Gradually add sweetened condensed milk, whisking until smooth. Stir in key lime juice and lime zest (if using) until well combined.
4 large egg yolks, 28 ounces sweetened condensed milk, 1 cup key lime juice, 2 tablespoons lime zest
If you turned off the oven, preheat it again to 350°F (175°C).
Pour filling into cooled crust. Bake for 15-20 minutes until filling is set but slightly jiggly in the center. Cool to room temperature for about an hour, then refrigerate for at least 2-4 hours (6-8 hours is best).
In a large bowl, beat heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form. Spread or pipe over chilled pie.
1 cup heavy whipping cream, 3 tablespoons powdered sugar, 1 teaspoon vanilla extract
Serve garnished with extra lime zest or lime slices if desired.
Notes
The calories listed are an approximation based on the ingredients in the recipe card and a serving size of 1 slice of pie. Actual calories will vary.
I use a full cup of key lime juice, which gives this pie a wonderfully tangy and slightly tart flavor. If you prefer a less tangy and sweeter taste, you can use as little as ¾ cup of key lime juice.