In a medium bowl, combine the graham crackers and sugar.
Add the melted butter and mix together until combined.
Press the mixture into the bottom and side of a pie pan, forming a neat border around the edge. Bake the crust until set and golden, 8 minutes. Set aside on a wire rack; leave the oven on.
Key Lime Filling
In a stand mixer, beat the egg yolks and lime zest at high speed until very fluffy, about 5 minutes.
Gradually add the condensed milk and continue to beat until thick, 3 or 4 minutes longer.
Lower the mixer speed and slowly add the lime juice, mixing just until combined, careful not to overmix. Pour the mixture into the crust.
Bake for 10 minutes, or until the filling has just set. Cool on a wire rack, then refrigerate. Freeze for 15 to 20 minutes before serving.
Whipped cream
In a stand mixer bowl using the whisk attachment, whip together heavy cream, powdered sugar and vanilla extract. Keep whisking until stiff peaks are just about to form. Make sure not to over-beat. Decorate the top of the pie.