Wash the fruit thoroughly in warm water. Using a vegetable peeler, remove the zest in strips about 1" wide, avoiding the bitter white pith. (If your peeler cuts too deeply, lay the strips, pith side up, on a counter and scrape off any pith with the blade of a knife.) Cut strips lengthwise into slivers the width of a wooden match (about 2-3mm).
Place the peel into a small saucepan and cover with water by 1-2" (this is NOT the water called for in the ingredients). Bring to a boil and cook until soft and translucent, 5-6 minutes. Drain.
Bring 2 c water, sugar and optional corn syrup to a boil in the same saucepan. Add the drained peel, reduce heat to a gentle boil, and cook until peel is translucent, about 20 min (a candy thermometer should read 210°F).
Let cool, then transfer to a tightly sealed jar and refrigerate.