01 - Place cranberries, granulated sugar, and water in a medium saucepan. Add the orange peel if desired, then stir to combine.
02 - Set the saucepan over medium heat and bring to a gentle simmer. Stir occasionally and cook for 10 to 12 minutes, until most of the cranberries have burst and the mixture thickens.
03 - Remove from heat. For a smooth syrup, pour the mixture through a fine mesh sieve, pressing the berries firmly to extract all liquid. Discard the solids.
04 - Stir in vanilla extract if using. Allow to cool completely, then transfer to a sealed bottle or jar. Refrigerate for up to 2 weeks.