Old-School Tiramisù

Lightly oil a 9x5" loaf pan and line with plastic wrap, leaving plenty of overhang. Whisk coffee, rum, salt, and 1 Tbsp. sugar in a shallow dish until sugar and salt dissolve. Chill until ready to use.
Whisk cream and mascarpone in a medium bowl just until smooth. Set aside.
Pour water into a large saucepan to come 2" up the sides and bring to a simmer. Combine egg yolks, remaining 1/3 cup sugar, and 2 Tbsp. water in a medium heatproof bowl. Set bowl over pan and reduce heat to low so water is at a bare simmer. Using an electric mixer (or a large whisk and a friend to fill in when your arm cramps), beat mixture on medium-high until eggs triple in volume and are extremely pale yellow and mixture forms a slowly dissolving ribbon when drizzled over itself in bowl, 5–7 minutes. Remove bowl from heat.
Using clean beaters (or whisk), beat reserved cream mixture until medium peaks form. Add egg yolk mixture and gently fold until no streaks remain.
Working one at a time, dip ladyfingers into chilled coffee mixture and let soak just long enough for them to soften, 1–2 seconds per side. Lay 8 cookies in prepared pan (it’s okay to smush them in a bit). Spread one-third of filling over. Layer another 8 soaked ladyfingers on top and spread half of remaining filling over. Repeat layers one more time with remaining ladyfingers and filling. Cover pan with plastic wrap; chill tiramisù until firm, at least 6 hours and up to 12 hours (longer is better; you’ll get cleaner slices and the layers will be fully set).
Invert a small platter or a large plate over top of loaf pan, hold together tightly, and flip them over so plate is underneath. Set on a work surface and carefully remove pan. Peel away plastic wrap from tiramisù; discard. Generously dust tiramisù with cocoa powder. Serve in slices, dusting with more cocoa powder if desired.
Do Ahead: Tiramisù (without cocoa powder) can be made 3 days ahead. Keep chilled.