Key lime pie is very well known, but many people are unsure about what 'key lime' refers to. Well, key limes are tiny limes that grow in Florida (the name probably comes from the Florida Keys). But don't worry if you can't get them, normal limes will do fine, and most people can't tell the difference.
This recipe makes enough for a pie baked in a 22cm (9 inch) diameter tin. Use a tin with a removable bottom - it will make your life a lot easier.
Grease the inside of your tin with butter, and spread most of the crumbs in an even layer on the bottom. The crumbs need to be compressed to make them hold together - I do this by pressing down on them with a potato masher. Now press the remaining crumbs up against the inside edge of the tin to form a rim. Again, you'll need to press the biscuit crumbs quite firmly to make them hold together.
Put the 'pie crust' into the fridge for at least half an hour.
Beat the egg yolks together in a large bowl. Stir in the condensed milk, mix well, and then add the lime juice and half of the zest. Stir until thoroughly mixed.