In a medium bowl, whisk the egg yolks until smooth and slightly lightened in color. This takes about 1-2 minutes of vigorous whisking. Gradually pour in the sweetened condensed milk while continuing to whisk – this prevents the mixture from becoming lumpy and ensures a silky texture.

Slowly incorporate the key lime juice and zest, whisking constantly. The mixture will begin to thicken naturally due to the acid reacting with the condensed milk. Add a pinch of salt to enhance the lime flavor. The filling should have a pale yellow-green tint and coat the back of a spoon.

Pour the lime mixture into your graham cracker crust, using a spatula to spread it evenly to the edges. Gently tap the pie pan on the counter to release any air bubbles. Cover with plastic wrap, ensuring it doesn't touch the surface, and refrigerate for at least 4 hours or overnight.

Just before serving, whip the heavy cream with powdered sugar until soft peaks form. Don't overwhip – you want it creamy, not stiff. Spread or pipe the whipped cream over the chilled pie.