Put the sheets of fish glue in a bowl and cover them with warm water to soften. Set aside the best looking cherry and dice the rest with the citron. Shred the baking chocolate.
Next, make the chocolate sauce:
Melt the butter over low heat and stir in the cocoa and water. Slowly stir in the sugar and continue cooking over a low flame for five minutes, stirring gently, then remove it from the fire.
1 tablespoon cocoa powder
1/2 cup sugar
3 tablespoons water
1 tablespoon butter
Squeeze the sheets of fish glue dry, and melt them in a pot over a low flame with three tablespoons of water, then let the mixture cool (if you are using powdered gelatin mix it up).
Cut the pan di spagna into quarter-inch thick slices and line the bowl with them. Sprinkle them with the liqueur, and, if need be, a little milk. Whip the cream to soft peaks, then beat in the confectioner's sugar and the gelatin, which should still be warm. Carefully divide the whipped cream mixture, and fold into one half the chocolate sauce.
Pour it into the bowl and sprinkle it with half the candied fruit. Fold the remaining diced candied fruit and the shredded chocolate into the other half of the whipped cream, and use it to fill the mold. Cover the top of the zuccotto with the remaining slices of pan di spagna, sprinkle them with a little milk, and chill for 5 hours.
Just before serving the zuccotto dip the mold in hot water. Unmold it onto a serving platter, decorate it with the cherry you set aside, and serve.