In a medium-sized saucepan, whisk together sugar, eggs, lemon zest, and salt until light and airy.
Whisk in lemon juice until well combined, then add cubed butter.
While continuously whisking, cook mixture over low heat until butter fully melts and bubbles start to form. Allow the mixture to simmer for about 10 seconds then remove from heat. It should be thick enough to coat a spoon at this point.
Using a fine mesh strainer, strain mixture into a medium-sized bowl and discard the solids. Mix in vanilla and allow mixture to come to room temperature before transferring to an airtight container. Store in the fridge for up to one week.
