Slice the oranges into wedges, then remove the flesh, leaving only the peel. Eat or use the orange flesh in another recipe. Remove most of the white pith with a knife, but leave a little bit of the pith for texture.
Boil the peels in a pot with enough water to cover the peels for 10 minutes, then change the water and soak 8 hours or overnight to remove the bitter taste.
Drain and transfer the peels into a pan. Add sugar, orange juice, and vanilla (optional). Stir and simmer over medium-low heat until the syrup thickens (about 25 minutes).
Let the peels dry on a wire rack for about an hour and coat with more sugar.
They can stay good in a sealed jar at room temperature for three weeks.
Prep and cook time: 45 minutes
Total time: 8+ hours
