Slice the oranges thinly, approximately 1/8 inch thick, picking out the seeds as you go. A mandolin makes this process move quickly.
Place the orange sliced in a medium saucepan over medium heat.
Stir in the water, sugar and vanilla.
Bring the mixture to a boil, then turn the heat to low.
Continue to simmer, stirring occasionally, until the mixture has darkened in color and thickened to a jammy consistency. The process will take approximately 35-40 minutes and the temperature will read 222-223 F on a candy thermometer.
Carefully transfer the marmalade to canning jars and seal with the lid.
Marmalade can be stored in the refrigerator for up to 10 days in an airtight container.
Notes
Set check: Marmalade is ready at 222–223°For when it wrinkles on the freezer-plate test.
It thickens as it cools: Don’t judge final thickness while it’s boiling hot.
Prevent bitterness: Avoid excess pith (white layer) and don’t let the mixture scorch once sugar is added.
Stir more near the end: As it thickens, stir frequently to prevent burning.
If it’s too runny: Simmer a bit longer and re-test.
If it’s too thick: Warm gently and stir in water 1 Tbsp at a time until loosened.