Tartlet Shells:
In the bowl of your food processor, blend together the flour, icing sugar, and salt. Add the very cold butter cut into small cubes and mix until you get very fine crumbs.
Add the egg (preferably pre-whisked in a separate bowl). Slowly mix until a dough starts to come together.
Bring the dough together into a ball, place between two sheets of baking paper, and roll into a large disk about 3mm thick (1/8 inch). Place on a flat tray and leave in the fridge to chill and rest for at least 1 hour, or up to 24 hours.
Remove the baking paper sheets and cut out small rounds of pastry with a round or fluted cookie cutter, at least as wide as the opening of the muffin pan.
Gently slide each piece of dough into a muffin pan, pressing on the bottom and the edges to remove any air bubbles. Prick the bottom with a fork and place back in the fridge to rest and chill for at least 1 hour, or up to 24 hours.
Preheat your oven to 160°C/325°F and place the muffin pan in the freezer while the oven is preheating. Line each tartlet shell with a small piece of baking paper and fill with baking weights (beads, rice, or dry beans). Bake for 15 minutes, remove the weights and baking paper, then bake for another 15 minutes, or until golden and dry to the touch. Set aside to cool down completely.
Orange Curd Filling:
Place the orange juice, zest, egg yolks, sugar, and cornstarch in a small saucepan. Whisk to combine, then turn on very low heat. Keep stirring until the sugar has dissolved and the liquid starts to slightly thicken (3 to 5 minutes).
Add the cubed butter a little bit at a time, stirring until fully melted before adding more. Continue to cook on low heat for a few minutes while stirring until the orange curd has thickened. Pour through a fine mesh sieve to remove any lumps.
Pour the warm orange curd into the cooled tartlet shells. Gently tap them on a hard surface to remove any air bubbles, then place in the fridge to set for at least 2 hours (or overnight), or until set.
Chocolate Ganache:
Finely chop the cooking chocolate and place it in a large heat-proof mixing bowl.
Heat up the cream in a small pot until it starts to simmer. Pour half of the hot cream over the chopped chocolate and leave for 2 to 3 minutes without touching it. Using a stiff spatula, stir in small circular movements, starting from the center of the bowl, then making your way towards the edges until combined.
Pour the rest of the hot cream over the ganache (reheat it first if needed) and stir again in circular movements until all the chocolate has melted and you get a smooth and shiny ganache.
Place the ganache in a piping bag, cut off the tip, and pipe over the orange curd. Gently tap each tartlet to help the ganache spread out (or use a small offset spatula if needed). Alternatively, drizzle the chocolate over the curd for another design.
Place in the fridge to set for at least 1 hour.
Finish decorating with some slices of dehydrated oranges, orange zest, or shaved chocolate/chocolate curls. Leave at room temperature for about 30 minutes before serving for the best texture.