How to Make It
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.
In a glass measuring cup, microwave Nutella in 15-second increments, stirring after each, until smooth and softened, about 30 seconds total. Set aside.
In another medium bowl, thoroughly stir mascarpone and powdered sugar until smooth and no lumps remain.
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.
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.
Loosely cover tiramisu with plastic wrap and chill at least 6 hours before serving. Just before serving, sprinkle each jar with cocoa powder.
Make ahead: Up to 1 day, chilled.