01 - Set oven to 350°F and allow to fully preheat.
02 - In a medium bowl, combine graham cracker crumbs and granulated sugar. Stir in melted butter until evenly moistened.
03 - Press mixture firmly into the bottom and up the sides of a 9-inch pie dish. Bake for 8–10 minutes until crust is lightly golden. Remove and cool slightly.
04 - In a large bowl, vigorously whisk egg yolks with lime zest until slightly thickened, about 2 minutes. Stir in sweetened condensed milk and whisk until smooth. Gradually add key lime juice, mixing until fully combined.
05 - Pour filling into cooled crust and smooth surface. Bake for 10–12 minutes; small bubbles should appear on filling and center should be just set.
06 - Let cool to room temperature, then refrigerate at least 3 hours or overnight until thoroughly chilled and firm.
07 - Using an electric mixer or whisk, whip heavy cream, powdered sugar, and vanilla extract until soft peaks form.
08 - Spread or pipe whipped cream over chilled pie. Garnish with lime zest or thin slices if desired. Slice and serve chilled.