Start by thoroughly washing and drying four fresh lemons. Zest the lemons, then cut in half and juice them, removing seeds.

In a medium saucepan, combine the lemon juice, zest, sugar, and water. Use a whisk to mix until sugar begins to dissolve. Add vanilla if desired.

Place the saucepan over medium heat, stirring continuously. Once it boils, reduce to a low simmer and cook for 30–40 minutes until thickened.

To test doneness, scoop a spoonful onto a cold plate. If it holds its shape, it's ready. If not, continue simmering.

Remove from heat and let cool slightly. Pour into sterilized jars and seal tightly. Allow to cool completely.