In bowl of standing mixer fitted with whisk attachment, beat yolks at low speed until just combined. Add sugar and salt and beat at medium-high speed until pale yellow, 1 1/2 to 2 minutes, scraping down bowl with rubber spatula once or twice. Add mascarpone and beat at medium speed until no lumps remain, 30 to 45 seconds, scraping down bowl once or twice. Transfer mixture to large bowl and set aside.

In now-empty mixer bowl (no need to clean bowl), beat cream at medium speed until frothy, 1 to 1 1/2 minutes. Increase speed to high and continue to beat until cream holds stiff peaks, 1 to 1 1/2 minutes longer. Using rubber spatula, fold one-third of whipped cream into mascarpone mixture to lighten, then gently fold in remaining whipped cream until no white streaks remain. Fold in melted chocolate. Set mascarpone mixture aside.

Working one at a time, drop half of ladyfingers into chocolate simple syrup mixture, roll, remove, and transfer to 13 by 9-inch glass or ceramic baking dish. Submerge ladyfingers in chocolate syrup, so it soaks up the liquid; entire process should take no longer than 3 seconds for each cookie. Arrange soaked cookies in single layer in baking dish, breaking or trimming ladyfingers as needed to fit neatly into dish.

Spread half of mascarpone mixture over ladyfingers; use rubber spatula to spread mixture to sides and into corners of dish and smooth surface. 

Repeat dipping and arrangement of ladyfingers; spread remaining mascarpone mixture over ladyfingers and dust with remaining 1 1/2 tablespoons cocoa. Wipe edges of dish with dry paper towel. Cover with plastic wrap and refrigerate 6 to 24 hours.