Instructions:
- Combine the sugar and water (and optional zest) in a small, heavy-bottomed saucepan over medium heat.
- Stir continuously until all the sugar is completely dissolved and the liquid is clear. Do not allow it to boil yet.
- Once dissolved, stop stirring. Increase heat slightly and bring the mixture to a gentle simmer.
- Cook to the desired consistency: Simmer for 2-3 minutes for thin syrup (glaze), 5-7 minutes for medium syrup (soaking), or until 220°F (104°C) for thick syrup.
- Remove the pan from the heat. If using vanilla extract, stir it in now after a 1-minute cooling period.
- If necessary, pour the syrup through a fine-mesh sieve into a clean container and allow to cool completely before storing or using.