Start by bringing a pot of water to a boil. Prepare a separate bowl with ice water and set it aside.
Slice the orange into 1/8-inch thick slices, ensuring evenness with the help of a mandoline.
Boil the orange slices for 2 minutes, then transfer them immediately to the ice water. Once cooled, pat them dry.
Return the pot to the heat (preferably heavy stainless steel). Add sugar and water over medium heat, attaching a thermometer to the pot. Stir until the sugar dissolves and bring the mixture to about 220°F.
Add in the orange slices, gently stirring to ensure they are all submerged in the syrup. Increase the syrup temperature to 250°F, stirring occasionally. This may take 20-30 minutes. Once at 250°F, remove from heat and transfer to a silicone baking mat to cool completely. For the best results, let them dry at room temperature overnight.
Melt chocolate using a double boiler or microwave. Dip each slice in chocolate, sprinkle with a bit of flaky salt (optional), and let it set. Properly stored, these treats can last for a month at room temperature or several months in the fridge or freezer.