Preheat the oven to 350°F.

In a medium bowl, stir together graham cracker crumbs, 6 tablespoons of melted butter, and sugar until well moistened. Pinch the crumbs between your thumb, pointer and middle fingers. If they feel moist and stick together when released from the pinch, they are ready to be pressed in the pan. If they feel dry, sandy, and lose their shape when released from the pinch, stir in more butter, 1 tablespoon at a time.

Add the crumb mixture to the pie plate. Using the flat bottom of a measuring cup or glass, firmly press the mixture into the bottom of the pan and up the sides of the pan, mounding the edge slightly above the rim of the pie plate. Bake the crust for 10 minutes, until dry and golden brown.

In a large bowl, combine the sweetened condensed milk, key lime juice, egg yolks, and zest. Whisk to combine.

Pour the filling into the warm crust and give it a jiggle to level things out. Bake at 350°F for about 10 minutes. Cool on a wire rack for about 20 minutes, then chill for at least an hour.

Place a large mixing bowl or the bowl of a stand mixer along with the beaters or whisk attachment in the fridge or freezer until well chilled, at least 20 minutes.

In a large, chilled bowl, combine the cream, sugar, and vanilla. With a handheld mixer (or in a stand mixer with a whisk attachment), beat on medium speed until frothy and starting to thicken, about 2 minutes. Increase the speed to medium-high and beat until soft to medium peaks form, 1 to 2 more minutes.

Top the pie with dollops of whipped cream, lime zest, and slices if you like, and serve.