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1
If using frozen cherries, thaw, reserving juices.
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2
Simmer cherries, 1/2 cup sugar, and liqueur in a 2- to 3-quart heavy saucepan, uncovered, stirring occasionally, until cherries are soft, about 5 minutes.
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3
Whisk together cornstarch and water until combined, then whisk into cherry mixture and boil 1 minute.
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4
Transfer to a shallow bowl and chill, uncovered, 1 hour.
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5
Pulse 1 cup almonds in a food processor until finely ground (do not pulse to a paste).
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6
Transfer to a bowl and stir in wafer crumbs and butter with a fork until combined well.
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7
Pat crumb mixture evenly onto bottom and 1 1/2 inches up side of buttered springform pan, then freeze while making filling, about 30 minutes.
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8
Beat egg whites with remaining 1/3 cup sugar, cream of tartar, and a pinch of salt in a medium metal bowl set over a saucepan of simmering water using a handheld electric mixer at medium-high speed until whites just hold soft peaks and instant-read thermometer registers 170F, about 5 minutes.
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9
Remove bowl from saucepan and continue to beat whites until they just hold stiff peaks, about 2 minutes more.
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10
Beat cream with Marsala in another bowl at medium speed until it just holds stiff peaks.
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11
Fold in ground amaretti and half of whites gently but thoroughly.
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12
Fold in remaining whites and pour into crust in pan, smoothing top.
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13
Drain cherries in a sieve set over a bowl and reserve juices.
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14
Scatter cherries evenly over top of tortoni, then swirl cherries into tortoni with tip of a sharp knife for a marbled effect.
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15
Sprinkle top of tortoni with remaining 1/2 cup toasted sliced almonds and freeze, loosely covered, until firm, at least 4 hours.
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16
Let stand in pan at room temperature 10 minutes to soften slightly before serving.
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17
Carefully remove side of pan, then cut tortoni into wedges and serve with cherry juices.
