Combine the coffee and sugar in a small saucepan. Bring to a simmer, stirring occasionally to dissolve the sugar. Remove from heat and stir in the eggnog. Set aside to cool completely.

In a stand mixer fitted with the whisk attachment, whip the heavy cream until stiff peaks form. Transfer whipped cream to a separate bowl, return bowl to the mixer, and replace whisk attachment with paddle attachment.

To the mixer, add the mascarpone cheese, sugar, eggnog, vanilla, nutmeg, and salt. Beat on high until combined. Add about a cup of the whipped cream and continue beating until incorporated. Gently fold in the rest of the whipped cream.

One at a time, quickly dip the ladyfingers in the coffee syrup until soaked but not soggy; arrange in an even layer along the bottom of an 8-inch square dish. (Line the ladyfingers up in two straight rows of six.**) Spread half of the the mascarpone filling on top of the ladyfingers. Place another layer of coffee syrup-soaked ladyfingers on top of the filling.

Spread half of the remaining filling on top of the ladyfingers. Transfer remaining filling to a pastry bag fitted with a large round tip (or a gallon ziplock bag with the corner cut off), and pipe the filling evenly over the tiramisu.

Cover with plastic wrap and refrigerate for at least 6 hours before serving. Just before serving, place cocoa powder in a fine mesh sieve and shake a light dusting over the surface. Cut into squares and serve.