01 - Preheat the oven to 175°C (350°F).
02 - In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until the texture resembles wet sand.
03 - Press the mixture firmly into the bottom and up the sides of a 23 cm (9-inch) pie dish.
04 - Bake the crust for 8 minutes. Remove and let cool slightly.
05 - For the filling, whisk egg yolks and lime zest in a bowl for 2 minutes until slightly thickened. Add sweetened condensed milk and whisk until smooth. Gradually whisk in key lime juice until fully combined.
06 - Pour the filling into the prepared crust and smooth the top.
07 - Bake for 12 minutes, until the filling is just set but still slightly wobbly in the center. Remove from the oven and cool to room temperature.
08 - Refrigerate the pie for at least 3 hours (preferably overnight) until thoroughly chilled.
09 - For the topping, whip the heavy cream with powdered sugar and vanilla until soft peaks form.
10 - Spread or pipe whipped cream over the chilled pie before serving.