Rinse the oranges well and cut the top and bottom off of each orange. Score the skin of each orange into quarters; peel the oranges, keeping the pith with the peel. Cut the peel into thin strips, about ¼ inch wide. Place peels in a sauté pan, cover with cold water and bring to a boil. Drain and repeat two more times.
While the peels are boiling, whisk together sugar and ½ cup water. Pour into a small saucepan and bring to a simmer over medium heat. Cook 5-6 minutes until sugar is dissolved. Add peels and return to a boil. Reduce heat and simmer until peels are very soft and translucent, about 45 minutes. Don't stir the peels while cooking as it can cause the sugar to crystalize; swirl the pan if necessary.
Remove the peels from the syrup and place on a drying rack. Dry for 4-5 hours. If desired, reserve any remaining syrup.
When dried, chop orange peel into small pieces; you'll need about ¼ cup for the recipe. Reserve remaining candied peels for another use; store in airtight container.
For the bark:
Place 8 ounces of the chips and butter in a medium heat-safe bowl. Place the bowl over a saucepan filled with about 2 inches of water; bring to a simmer over medium heat. Keep the water at a gentle simmer, don't boil.
Stir the chips until melted and smooth. Add the remaining chocolate about 2 ounces at a time, stirring until smooth after each addition.
Remove the bowl from the pan and wipe the sides of any excess moisture. Pour chocolate onto a jelly roll pan lined with parchment paper; use a curved spatula to smooth the mixture to about ¼ inch thick. Add pistachios and dried cranberries; lightly press into chocolate. Add candied orange peel. If desired, sprinkle with sea salt.
Place in refrigerator for 15-20 minutes to set; continue cooling at room temperature until the bark is fully set. Break into pieces and store in an airtight container for up to a week.
The number of pieces you end up with will vary depending on how large or small you break the pieces of chocolate. I break it randomly, so the pieces vary in size. That method should easily yield you 28 to 36 pieces.