For the chocolate crostata:
1) Line a heavy large baking sheet with parchment paper. Mix the flour, cocoa powder, sugar, orange peel, and salt in a food processor. Add the butter. Blend until clumps form. Add two tablespoons of iced water and blend. Add more iced water, one tablespoon at a time, until the dough forms. Turn the dough out onto a work surface and knead briefly to form a smooth dough. Gather the dough into a ball then divide the dough into 8 equal pieces. Flatten each piece into a disk.
2) Press one dough piece on the prepared baking sheet into a 11cm round. Spoon one generous tbsp of the marmalade into the centre of the round. Fold the dough border in to form a 7 1/2cm round and allow the centre of the crostata to remain uncovered. Pleat loosely and pinch the dough to seal any cracks. Repeat with the remaining dough pieces and marmalade, spacing the crostatas evenly apart. Cover and refrigerate the crostatas until they are cold, at least 45 mins and up to 4 hrs.
3) Preheat the oven to 190°C/ gas mark 5.
4) Bake the crostatas until the crusts are set, about 18 mins. Cool for 10 mins. Slide a metal spatula under the crusts to free the crostatas from the baking sheet. Garnish with the crystallised orange peel and serve warm.
For the crystallised orange peel:
1) Using a vegetable peeler, cut the orange part of the peel from the stem end of the orange down to the navel end, forming long 2 to 2 1/2cm-wide strips. Bring a heavy small saucepan of water to a boil. Add the peels and cook for 1 min. Drain and then rinse the peels under cold water. Repeat cooking the peels in the saucepan with fresh boiling water and rinsing under cold water.
2) Stir the sugar and 120ml of fresh water in a heavy small saucepan over medium-high heat until the sugar dissolves. Bring to a boil. Add the orange peels and simmer over medium-low heat until tender, about 15 mins. Using tongs, transfer the peels to a sheet of parchment paper to dry slightly, about 1 hour.
3) Line a small baking sheet with parchment paper. Stir the chocolate in a small bowl set over a saucepan of simmering water until melted and smooth. Dip 4cm of each candied orange peel into the chocolate then place them on the prepared baking sheet and refrigerate until the chocolate is set, about 15 mins.