Begin by thoroughly washing and drying your fresh lemons. Zest the lemons using a microplane or grater, avoiding the white pith. Cut and juice the lemons, removing any seeds.

In a medium saucepan, mix together the lemon juice, zest, granulated sugar, and water. Stir in the vanilla extract if desired and mix well.

Place the saucepan over medium heat, stirring until the sugar dissolves. Bring to a boil, then reduce heat and let it simmer for 30–40 minutes, stirring occasionally.

To check doneness, place a spoonful on a cold plate. If it wrinkles when pushed with a finger, it’s ready. If not, simmer longer and test again.

Remove from heat, let cool for 5 minutes, then pour into sterilized jars and seal. Allow to cool completely before refrigerating.