
In the winter months, there's nothing like the bewitching, slightly exotic flavor of Key limes or the larger regular limes (a.k.a. Persian) to transport you mentally to some warm tropical spot, preferably an island with an endless beach. This cake, with its tart lime filling offset by billowy drifts of sweet, marshmallow-like frosting, should do the trick.
Key limes are smaller than Persian limes and often come packed in a net bag, but don't despair if you can't find them. Persian limes make an excellent substitute (see Cooks' Notes, below, for more details on buying limes). Another helpful hint: Because there's grated zest in both the cake and the filling, grate all the zest you need first, before you start juicing.
•Limes, whether Key or Persian, can vary tremendously in the amount of juice they hold; some can be quite dry and pulpy inside, while others will release plenty of liquid. That makes it difficult to give you, the cook, an idea of how many limes you'll need. We think you'll be safe buying a total of 1 1/2 pounds Key limes or 10 medium Persian limes to make the cake layers, filling, and icing. But don't be surprised if you have extra. Use them up in a batch of margaritas!
•Cake layers (not split) can be made 1 day ahead and chilled, wrapped individually in plastic, or frozen 1 week. Halve chilled layers while still cold, but defrost frozen layers before attempting to halve them. Filling can be refrigerated, covered, overnight. Stir before using.
•The egg whites in the frosting are not cooked. If this is a concern, you can substitute reconstituted dried egg whites, such as Just Whites. Cake can be assembled and frosted 2 hours ahead if using fresh whites, and 6 hours ahead if using dried whites. Alternatively, cake can be filled but not frosted 1 day ahead. Wrap well in plastic and chill. Let stand at room temperature 1 hour before frosting it.
