1First, make the crust. Add the butter, flour, sugar, and salt to a food processor. Pulse until a sandy mixture forms and you can no longer see pieces of butter.
2Put the processor on medium high speed and stream in the cold water until a ball forms that pulls away from the sides.
3Transfer the dough to a lightly floured work surface and divide in 2 pieces. Wrap 1 piece in plastic wrap and freeze for later use. The other piece, roll out flat and transfer to a greased and floured 26 by 26 centimeter baking dish. Refrigerate until firm.
4Preheat the oven to 180 degrees Celsius.
5When the crust is firm, pierce the bottom repeatedly with the tines of a fork. Place a piece of parchment paper on the bottom and fill the bottom with dried beans so the dough does not rise during baking. Bake 15 minutes. Remove and cool.
6Chunk the chocolate into a microwave safe bowl and microwave in 1 minute increments until melted. Once the crust cools, baste the bottom and sides of the crust with a light layer of chocolate. Let the chocolate set at room temperature.
7While the crust cools, make the orange curd.
8In a food processor or coffee grinder, blend the sugar and orange zest together.
9Set up a double boiler and add into the top pot the sugar mixture, eggs, and strained orange juice. Stir constantly until the mixture thickens.
10Soak the gelatin in just enough water to rehydrate it. Add it to the orange curd mixing it in well.
11Remove from heat and when the curd has cooled to 40 degrees Celsius, add the butter cube by cube beating well after each addition.
12Pour the orange curd over the chocolate coated crust and tap the pan on the counter a few times to remove any bubbles. Refrigerate overnight.
13The next day, top the tart. Add the water and marmalade to a pot and bring to the boil. Stir to bring up the marmalade.
14Strain the marmalade and add the strained liquid to a clean pot and heat it up along with the gelatin previously rehydrated in a little water. Set aside to cool.
15Decorate the top of the tart with the mandarin pieces. Baste with the cooled marmalade mixture. Refrigerate until set and serve.