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1
Combine whole milk, cream, and vanilla pod in a heavy saucepan; heat to 75°C without boiling.
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2
Whisk egg yolks with caster sugar until pale and thick, approximately 3 minutes; temper by slowly adding hot milk while whisking constantly.
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3
Return mixture to saucepan and cook over medium-low heat to 82°C, stirring continuously with a wooden spoon until it coats the back; strain through fine sieve and chill completely, 2 hours minimum.
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4
Fold candied peel and chocolate into chilled custard; gently combine sponge cubes with maraschino and fold into custard mixture.
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5
Line a 2-litre domed mould with plastic wrap, press mixture firmly, fold plastic over top, and freeze 8 hours or overnight.
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6
Unmould onto serving plate; roll marzipan thin and drape entirely over cassata, sealing seams.
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7
Beat egg whites to stiff peaks, fold in royal icing sugar in three additions; pipe or spread white icing over marzipan in decorative peaks.
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8
Refrigerate 1 hour before serving; slice with hot knife dipped in water between cuts.