01 - Preheat the oven to 350°F.
02 - In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until evenly moistened.
03 - Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Bake for 8–10 minutes until lightly golden. Cool slightly.
04 - In a large bowl, whisk together sweetened condensed milk, egg yolks, key lime juice, lime zest, and green food coloring (if using) until smooth and fully combined.
05 - Pour the filling into the cooled crust and smooth the top.
06 - Bake for 12–15 minutes, just until the center is set but still slightly wobbly.
07 - Remove from oven and cool to room temperature, then chill in the refrigerator for at least 3 hours.
08 - Beat cold heavy cream, powdered sugar, and vanilla extract together with a mixer until soft peaks form.
09 - Spread or pipe the whipped cream over the chilled pie. Garnish with lime zest and lime slices as desired. Serve chilled.