Use a food processor to turn your graham crackers into fine crumbs. Pulse with the powdered sugar, salt, and browned butter until it looks like wet sand.
Press firmly into the bottom and sides of your tart pan. Bake for 10-15 minutes, and then cool completely. Turn the oven off for 30 minutes while you let the crust cool.
Key Lime Pie Filling
Turn the oven back on to 325ºF.
Zest about 5 key limes to get 2 tablespoons of zest, and juice about 20 key limes to create ½ cup of juice. Set aside.
Remove 4 egg yolks from the whites (save the whites for another recipe). Beat the zest into the yolks until the color goes from golden to pale yellow and the texture seems thicker.
Whisk in the can of sweetened condensed milk.
Whisk in the lime juice for roughly 5 minutes until it feels thicker.
Cover and rest for 15 minutes to let the air bubbles come up. Give it a quick fold with a spatula to release the air.
Pour your filling into your tart crust and gently spread it to the edges without getting crumbs in it. Give it a jiggle to smooth out the surface.
Bake for 15-20 minutes at 325ºF.
Cool completely, and then pop it in the fridge for 4 hours.
Whipped Cream
Whisk the powdered sugar and vanilla into your cold whipping cream. Continue whipping until it's thick and looks like whipped cream.
Move to a piping bag and decorate the surface of your cold key lime pie. You may also sprinkle with more lime zest or slices of limes.