In a saucepan, mix together lemon zest, lavender (if using), and granulated sugar together. Rub the zest and lavender into the sugar to help release the oils.
Add the salt and lemon juice and turn on the heat to medium-high. Stir well until the mixture is hot and simmering.
In a separate bowl, beat the eggs well.
Slowly add one-third of the lemon mixture to the egg, beating the eggs well as you pour.
Transfer the egg mixture to the saucepan, beating the mixture in the saucepan as you pour.
Continue to mix continuously until the mixture thickens.
Test doneness by sticking a wooden spoon into the mixture and running your finger down the back of the spoon. When the line stays where you ran your finger, it is ready.
Remove from heat and add in the butter, stirring until completely combined.
Strain to remove the zest, lavender, and any egg that may have cooked. Allow to cool completely and store in an airtight container in the refrigerator.
Notes
While stirring, scrape the sides and bottom of the saucepan to prevent sticking or burning. When cooling the curd, place plastic wrap directly on the curd to prevent skin from forming.