Step 1: Wash the oranges thoroughly. Using a sharp knife, slice the oranges into 1/8 to 1/4-inch thick rounds. Remove any seeds.
Step 2: Bring a large pot of water to a rolling boil. Add the orange slices and blanch for 5 minutes. Drain the oranges and repeat the blanching process one more time (for a total of two blanched cycles). This helps remove bitterness and tenderize the peel.
Step 3: In a wide, shallow pot or deep skillet, combine 2 cups granulated sugar and 2 cups water. Heat over medium heat, stirring until the sugar completely dissolves. Stir in the vanilla extract, if using.
Step 4: Carefully arrange the blanched orange slices in a single layer in the sugar syrup. Bring the syrup to a gentle simmer, then reduce the heat to low. Cook for 60 to 90 minutes, or until the white pith of the orange peel becomes translucent and the slices appear glossy and slightly softened. Ensure the syrup remains at a gentle simmer and doesn't boil vigorously.
Step 5: Using tongs, carefully transfer the candied orange slices to a wire rack set over a baking sheet. Allow them to dry at room temperature for at least 12-24 hours, or until they are no longer sticky to the touch and are slightly firm. This drying time is crucial for texture.
Step 6: (Optional) Once completely dry, you can toss the candied orange slices gently in a bowl with additional granulated sugar to give them a sparkling finish and prevent sticking. Store in an airtight container.