Combine the sugar, brandy or rum, and water in a small saucepan. Bring to a boil over moderate heat, stirring to dissolve the sugar. Remove from the heat, transfer to a shallow bowl, and let cool.

In a large bowl, stir the ricotta, vanilla, and 1/2 cup of the cooled syrup. Fold in 4 tablespoons of the grated chocolate.

Choose a dish that measures about 7 by 10 inches. One at a time, dunk half the ladyfingers in the brandy syrup to moisten them well, then arrange them in a single layer in the bottom of the dish. Spread half the ricotta mixture evenly over the ladyfingers. Moisten the remaining ladyfingers with the remaining syrup and arrange them over the ricotta. Top with the rest of the ricotta, spreading it evenly. Sprinkle the top with the remaining 2 tablespoons grated chocolate. Cover and refrigerate for several hours before serving.