Make the mousse: Turn the ricotta into a food processor and process until smooth. Set aside. Add the cold water to a small bowl and sprinkle the gelatin over it. Set aside. With an electric mixer, beat the heavy cream and vanilla until soft peaks form; refrigerate until ready to use. Toss the candied fruit, farro, lemon zest, and cinnamon (if using) together in a second small bowl and set aside.

With a balloon whisk, whisk together the egg, egg yolks, sugar, and salt in the top of a double boiler or in a heatproof bowl set over a saucepan containing 1 inch of simmering water (the bowl should fit snugly). Whisk until the egg mass is fluffy and warm and the whisk leaves a trail (the mixture should register 150 degrees on an instant-read thermometer), about 3 minutes. Remove the insert or bowl from the heat, add the softened gelatin, and whisk until dissolved. Whisk in the ricotta. Place the insert or bowl in a larger bowl filled with iced water and whisk until the mixture is cold and beginning to set, about 2 minutes. Fold in the whipped cream with a rubber spatula, and then return the bowl to the iced-water bath. Allow the mousse to chill, folding occasionally with the rubber spatula, until it sets enough that it will hold the farro aloft, about 5 minutes. Fold in the farro mixture until the bits are distributed evenly throughout. Turn the mousse into a shallow container or pipe or spoon it into 6 serving glasses. Cover with plastic wrap and refrigerate for 4 hours.

Make the caramel: Pour the sugar into a clean, heavy-bottomed 2-quart saucepan and set the pan over low heat. Cook, without stirring, until the sugar pools amber around the edges of the saucepan, 5 to 7 minutes. Maintaining low heat, stir the caramel with a wooden spoon until the sugar has completely melted and the caramel is rich, warm amber throughout. Remove the pan from the heat. Add half of the hot water and stir to dissolve the hardened caramelized sugar. Add the remaining hot water and stir. Transfer the caramel to a heatproof liquid measuring cup (you should have ⅔ cup) and let cool to room temperature. When cooled, the caramel should be viscous but still liquid. If it is too thick, thin it with a few drops of water.

If serving in glasses, drizzle caramel over each portion and sprinkle with pistachios. If serving on plates, drizzle a scant 2 tablespoons of caramel onto the center of each of 6 dessert plates. Using a hot spoon, scoop 2 quenelles (rounded egg-shaped spoonfuls) of chilled mousse onto each plate and sprinkle with pistachios. Serve right away.