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 2 tablespoons of ice water and blend. Add more ice water, 1 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.
Press 1 dough piece on the prepared baking sheet into a 4 1/2-inch round. Spoon 1 generous tablespoon of the marmalade into the center of the round. Fold the dough border in to form a 3-inch round and allow the center 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 minutes and up to 4 hours.
Preheat the oven to 375 degrees F.
Bake the crostatas until the crusts are set, about 18 minutes. Cool for 10 minutes. Slide a metal spatula under the crusts to free the crostatas from the baking sheet. Garnish with the candied orange peel and serve warm.
Candied Orange Peel:
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 3/4 to 1-inch-wide strips. Bring a heavy small saucepan of water to a boil. Add the peels and cook for 1 minute. 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.
Stir the sugar and 1/2 cup 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 minutes. Using tongs, transfer the peels to a sheet of parchment paper to dry slightly, about 1 hour.
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 1 1/2-inches 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 minutes.