Mix together the ricotta, chocolate, candied fruit and 4 tablespoons of the granulated sugar in a mixing bowl, and then spread evenly into the pie crust. Lay slices of sponge cake on top of the filling close to the crust. Heat 1/2 cup water in a saucepot, and then remove from the heat and stir in the remaining 1 tablespoon granulated sugar and the rum. Lightly coat the sponge cake with the rum water using a pastry brush. Place a plate on top of the cake and then invert and sprinkle with the icing sugar. Refrigerate 1 hour and serve.