Spray a 9 by 5 by 3-inch metal loaf pan
with nonstick spray. Line the pan with plastic wrap, allowing the excess to hang over the ends and sides.
Combine the crushed amaretti cookies and the melted butter. Put in the lined pan and press down to firm and form a crust.
Whisk 1/2 cup of the sugar, the egg
yolks, lemon juice, lime
juice, limoncello, and salt in a large metal bowl to blend
. Set the bowl over a saucepan
of simmering water (do not allow the bottom of the bowl to touch the water). Whisk
the egg mixture until it is thick and creamy, and a thermometer inserted into the mixture registers 160 degrees F, about 5 minutes. Set the bowl of custard into another bowl of ice water to cool completely. Stir in the zest
Using an electric mixer
, beat the cream and remaining 1/4 cup sugar
in another large bowl until firm peaks form. Using a large rubber spatula
, gently fold the whipped cream
into the custard
. Spoon the mixture onto the prepared crust
. Fold the overhanging plastic wrap over the custard and freeze until frozen, at least 8 hours or up to 3 days.
Unfold the plastic wrap. Invert the semifreddo
onto a platter and peel
off the plastic wrap
. Cut the semifreddo into 1-inch slices and sprinkle with crushed amaretti
cookies and serve.