Heat a large skillet over high heat. Add 1 1/2 bottles root beer and bring to a boil. Cook until syrupy and reduced to about 1/2 cup, 7 to 8 minutes. Cool completely, then cover and refrigerate.
Combine the mascarpone and 1 1/4 cups confectioners' sugar in a mixer fitted with the paddle attachment. Beat on medium just until smooth and light (do not overbeat). In another bowl, whip the cream and vanilla with the remaining 1/4 cup confectioners' sugar until it holds soft peaks. Fold about 1/4 of the cream into the mascarpone to lighten it, then fold in the remaining cream.
Pour the remaining 1 1/2 bottles root beer into an 8-inch square baking dish or other container for soaking the ladyfingers. Working with a few ladyfingers at a time, dip briefly in the root beer (just a few seconds to lightly saturate; don't oversoak) and line the bottom of a trifle dish. Spread a quarter of the mascarpone mixture in an even layer over top. Sprinkle with a quarter of the crushed root beer barrels. Repeat with 2 more layers each of soaked ladyfingers, mascarpone and crushed root beer barrels. Make 1 final layer of soaked ladyfingers and a smooth layer of mascarpone on the top. Reserve the remaining crushed root beer barrels. Cover and refrigerate the tiramisu until well chilled, about 4 hours.
When ready to serve, sprinkle the top of the tiramisu with the remaining crushed root beer barrels and drizzle with some root beer reduction. Scoop out portions into serving dishes and drizzle with the remaining root beer reduction.
Copyright 2017 Television Food Network, G.P. All rights reserved.