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1
Spray a 9 by 5 by 3-inch metal loaf pan with nonstick spray.
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2
Line the pan with plastic wrap, allowing the excess to hang over the ends and sides.
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3
Combine the crushed amaretti cookies and the melted butter.
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4
Put in the lined pan and press down to firm and form a crust.
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5
Whisk 1/2 cup of the sugar, the egg yolks, lemon juice, lime juice, limoncello, and salt in a large metal bowl to blend.
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6
Set the bowl over a saucepan of simmering water (do not allow the bottom of the bowl to touch the water).
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7
Whisk the egg mixture until it is thick and creamy, and a thermometer inserted into the mixture registers 160 degrees F, about 5 minutes.
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8
Set the bowl of custard into another bowl of ice water to cool completely.
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9
Stir in the zest.
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10
Using an electric mixer, beat the cream and remaining 1/4 cup sugar in another large bowl until firm peaks form.
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11
Using a large rubber spatula, gently fold the whipped cream into the custard.
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12
Spoon the mixture onto the prepared crust.
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13
Fold the overhanging plastic wrap over the custard and freeze until frozen, at least 8 hours or up to 3 days.
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14
Unfold the plastic wrap.
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15
Invert the semifreddo onto a platter and peel off the plastic wrap.
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16
Cut the semifreddo into 1-inch slices and sprinkle with crushed amaretti cookies and serve.
