Using a sharp knife, remove the bitter white pith from the orange peels so they are about 1/8 inch thick; cut the peels into 1/4-inch-wide strips. Transfer the peels to a saucepan filled with cold water; bring to a boil and boil the peels for 7 minutes. Drain. Repeat with fresh water, until the orange peels are tender, 15 minutes total. Drain.

Rinse out the saucepan. Add 2 1/2 cups of the sugar, the corn syrup and 2 cups of water and bring to a boil, stirring until the sugar is dissolved. Add the orange peels and simmer over low heat until slightly translucent, about 30 minutes.

Spread the remaining 1 cup of sugar in a pie plate. Using a slotted spoon, lift the candied peels from the syrup, letting the syrup drip back into the saucepan; transfer the peels to the pie plate. Toss the warm peels in the sugar to coat thoroughly. Let the peels cool completely in the sugar, tossing occasionally, about 30 minutes.

Line a cookie sheet with parchment paper. In a double boiler set over a pot of simmering water, heat the bittersweet chocolate until two-thirds melted. Remove from the heat. Stir the chocolate with a rubber spatula until it is completely melted and registers about 90 degrees on an instant-read thermometer.

Spread the warm chocolate on the parchment paper to a rough 9-by-13-inch rectangle. Working quickly, so the chocolate doesn't set, pick out the orange peels, reserving the sugar for another use. Scatter the orange peels and pistachios evenly over the melted chocolate; gently tap the cookie sheet on the work surface to flatten the chocolate and allow the toppings to sink in slightly. Refrigerate for about 15 minutes, just until firm. Cut or break the bark into 2-inch pieces and serve.