Place the orange half in the center of an 8-inch glass bowl with the cut side facing down. Place 2 forks on top of the orange to weigh it down, pour the water into the bowl, and freeze for 24 hours.
Temper the chopped semi-sweet chocolate over a water bath and pour the liquid into a 9-inch glass bowl. Take the smaller glass bowl out of the freezer and remove the ice dome from the bowl. Remove the orange half from the ice dome and dip the ice dome into the tempered chocolate until it is completely coated (make sure the hollow left from the orange half is also covered).
Flip the chocolate-coated ice dome over, place it on top of an upside-down casserole dish, and let the chocolate harden.
Once the chocolate dome has set, carefully remove it from the ice dome and flip it over so it's like a bowl.
Combine the cream cheese, sugar, and juice of 1 orange, gently fold in the whipped cream, and transfer the mixture to a piping bag.
Pipe some of the orange whipped cream into the bottom of the chocolate dome. Dip a couple ladyfingers in amaretto, break them up into smaller pieces, and place them on top of the orange whipped cream in the chocolate dome.
Cover the ladyfinger pieces with another layer of orange whipped cream and top with half of the Mandarin oranges. Dip 5-6 ladyfingers in amaretto and arrange them on top of the Mandarin oranges in one layer. Top with a layer of orange whipped cream, followed by the rest of the Mandarin oranges, and the final layer of the amaretto-dipped ladyfingers.
Flip the orange chocolate tiramisu dome over and refrigerate for 4 hours.
Shortly before serving, bring the orange juice and sugar to a boil in a saucepan over medium heat and let the mixture reduce by half. Remove the chilled orange chocolate tiramisu dome from the fridge and pour the hot orange glaze into the well on top, which will cause the chocolate to melt, revealing the delicious filling inside.