Heat the oven: Set to 350°F (175°C). You’ll bake the crust first, then the filled pie briefly.
Make the crust: Stir graham cracker crumbs, sugar, and melted butter until the texture feels like wet sand.
Press into a 9-inch pie dish, pushing up the sides. Use a flat-bottomed cup to compact it.
Bake the crust: Bake for 8–10 minutes, just until fragrant and slightly darker. Let it cool while you mix the filling.
Whisk the filling: In a bowl, whisk egg yolks and a pinch of salt until slightly thickened, about 30 seconds.
Add sweetened condensed milk and whisk until smooth. Stir in key lime juice and lime zest. The mixture will thicken a bit on its own.
Fill and bake: Pour the filling into the warm crust.
Bake at 350°F for 12–15 minutes, until the edges are set and the center has a small wobble. Do not overbake.
Cool and chill: Let the pie cool to room temperature, then cover and chill for at least 3 hours, preferably overnight. This sets the texture and deepens the flavor.
Whip the topping: Beat cold heavy cream with powdered sugar and vanilla until soft peaks form.
Spread or pipe over the chilled pie.
Garnish and serve: Add a sprinkle of lime zest or thin lime slices. Slice with a warm, dry knife for clean edges.