Pre heat the oven to 350F degrees.

In a food processor, pulse the crackers with the sugar until finely ground. Add the butter and process until combined.

Transfer the mixture to a 9-inch springform pan and press evenly into the bottom and sides. Bake for 10 minutes or until set and golden. Remove from oven and set aside to cool. Leave the oven on.

Using a stand (or electric) mixer, beat the egg yolks with the lime zest, at high speed, until pale and fluffy.

Lower the speed and gradually add the sweet condensed milk, continuing to beat until thick. Then, slowly add the key lime juice, beating just until combined.

Pour the filling into the crust and bake the pie for 10 minutes or until the filling is just set. Transfer to a wire rack to cool.

Increase oven's temperature to 400F.

Using the stand (or electric) mixer, beat the egg whites and the cream of tartar until soft peak forms. Gradually add the sugar and beat until stiff, glossy peaks form, adding the vanilla extract at the end.

Scoop the meringue on the pie, making swoops and swirls with the back of a spatula.

Bake the pie again for about 5 minutes or until the meringue is set and browned in spots. Be careful, as the browning can happen pretty quickly.

Remove the pie from the oven and chill it for 2 to 3 hours before serving.