In an 8-by-11-inch baking dish, lay enough ladyfingers in a single layer so that the bottom is covered. With a pastry brush or spoon, evenly coat the ladyfingers with 1/2 of the hot chocolate.
In a large mixing bowl, whisk together the mascarpone, melted chocolate, 1/4 cup powdered sugar and 1 1/2 teaspoons vanilla just until combined. Spread mixture evenly over the soaked ladyfingers. Place remaining ladyfingers in a single layer over the mascarpone mixture and soak with the remaining hot chocolate.
Using a mixer, whip the cream with the remaining 1/4 cup powdered sugar, cocoa powder and remaining 1 1/2 teaspoons of vanilla. Spread evenly over the soaked lady fingers.
Chill in the refrigerator for at least 1 hour and up to 2 hours. Dust with cocoa powder and garnish with chocolate shavings before serving.