Can I use regular limes instead of key limes?
Yes. Persian limes (the common grocery store kind) work great. Key limes are a little more floral and aromatic, but this pie will still taste like a classic either way. If you use regular limes, do not skip the zest since it boosts that fresh lime snap.
Why does the filling thicken without flour or cornstarch?
Two things do the heavy lifting: the condensed milk thickens when combined with acidic lime juice, and the egg yolks set into a custard as the pie bakes. It is a very satisfying little bit of kitchen science.
How do I know when the pie is done?
The center should still have a small jiggle like set gelatin, not a slosh. The edges will look more set than the middle. It will finish firming as it cools and chills.
My pie cracked on top. Did I ruin it?
Not at all. A crack usually means it baked a touch long or cooled too quickly. Cover it with whipped cream and call it intentional. Next time, pull it when it is still slightly jiggly in the center.
Can I make it ahead?
Absolutely. Key lime pie is happiest after a long chill. Make it 1 day ahead for clean slices and best flavor.
Are the egg yolks fully cooked?
Yes. This recipe bakes the filling until it is set around the edges with a gentle wobble in the center, then it continues to firm up as it cools and chills.
How much zest is 1 tablespoon?
As a rough guide, 1 tablespoon loosely packed zest is about 2 to 3 Persian limes, or more like 6 to 8 key limes since they are smaller. Zest first, then juice.
Can I use a store-bought crust?
Sure. A store-bought graham crust works in a pinch. The filling bake time stays about the same, but keep an eye on the edges so the crust does not over-brown.