Use full-fat block cream cheese for the smoothest and most stable filling.
Make sure it is fully softened before mixing.
Key Lime Juice
Fresh key lime juice gives the most authentic flavor, but regular lime juice works if key limes are unavailable.
Sweetened Condensed Milk
One can gives a more tart filling.
If you prefer a sweeter flavor, add another half can, about 7 ounces.
Lime Zest
The zest adds strong lime aroma without making the filling more acidic.
Expert Tips for Best Results
Line the Pan Well
Use two strips of parchment paper crossed over each other, leaving about an inch of overhang.
This makes it much easier to lift the frozen slab out of the pan.
Cool the Crust Before Filling
If the crust is still hot, the filling can become too loose around the edges.
Let it cool before adding the lime mixture.
Beat the Cream Cheese First
Beat it alone until completely smooth before adding the condensed milk.
This prevents lumps.
Taste Before Adding More Condensed Milk
Start with one can.
If you like a sharper, more tart key lime flavor, keep it that way.
Freeze Long Enough
At least 4 hours is important if you want clean slices.