In a medium bowl, combine graham cracker crumbs and sugar.
Add melted butter and mix thoroughly.
Divide evenly into individual pans.
Press across the bottom and up the sides.
Bake for 5 minutes.
Remove from oven and allow to cool.
Filling
Whisk together sugar, egg yolks, lime juice, and lime zest in a medium heat proof bowl.
Place over a pot of simmering water.
Cook and stir until curd reaches 180 degrees.
Remove from heat and strain through a fine mesh sieve.
Whisk in butter.
Divide evenly into prepared crusts.
Cover with plastic wrap, making sure it touches the top of the curd.
Refrigerate until cool.
Notes
Substitutions:
Don't have access to key limes? Use regular limes instead!
Expert Tips:
Make sure there are no large chunks of graham cracker in the crumbs.
A tart tamper or small measuring cup will greatly help when pressing down the crust. Just remember, it's a little crumbly, so it might fall off the side once or twice when you're pressing.
The Graham cracker crumbs absorb the butter pretty quick so, try and work as fast as you can!
When making the custard, the temperature is more than likely to go up and down. Don't worry! As you stir, the air cools it down.
The custard needs to be at 180 degrees. The consistency will be a lot like jello! One of the best tests you can do to see if it's properly done is pick up a little bit of the filling, place it back on top, and see if it melts back into the custard or stays separate. If it stays separate, it's ready!
Straining the custard is a very important step. It's strains out any cooked egg, zest, or seeds that might be in the custard.
Bury the butter into the custard and let it melt. It's the easiest way! Just use a whisk to mix it in.
Another way to get the custard into the shells is to cover the custard with plastic wrap and put into the fridge and let it cool completely. From there, you can pipe it into the tart shells.