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1
Put egg yolks, granulated sugar, and salt in a medium bowl and set over a pan with 1/2 in. barely simmering water over low heat. Cook, beating with a hand mixer, until pale and thickened, about 4 minutes. When you lift beaters, mixture should fall slowly and form a ribbon that holds its shape for several seconds. Remove bowl from pan of water and set aside.
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2
In a glass measuring cup, microwave Nutella in 15-second increments, stirring after each, until smooth and softened, about 30 seconds total. Set aside.
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3
In another medium bowl, thoroughly stir mascarpone and powdered sugar until smooth and no lumps remain.
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4
In a large bowl with a mixer, beat whipping cream until stiff. Fold two-thirds of whipped cream into mascarpone mixture. Add about 1/2 cup remaining whipped cream and the Nutella mixture to egg yolk mixture and fold gently to combine, then fold in all remaining whipped cream.
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5
Spoon a 1/2-in. layer of Nutella mixture into bottom of each of six 1/5-liter jars. Put espresso and Frangelico, if using, in a small bowl and dip 2 ladyfinger halves in it; layer them, overlapping on top of Nutella mixture in one jar. Repeat for five more jars. Spoon a 1/2-in. layer of mascarpone cream into each jar. Add another layer of soaked ladyfinger halves, followed by a 1/2-in. layer of Nutella mixture. Add another layer of soaked ladyfinger halves, then a final layer of mascarpone cream.
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6
Loosely cover tiramisu with plastic wrap and chill at least 6 hours before serving. Just before serving, sprinkle each jar with cocoa powder.
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7
Make ahead: Up to 1 day, chilled.
