In a heatproof bowl, cream the butter and sugar till well combined.
Add the eggs, 1 at a time, and then add the lemon juice and lemon zest.
Fill the bottom pot of your Bain-marie (double boiler) with 2 inches of water.
Once the water begins to simmer, place the bowl of lemon + egg mixture onto the bain-marie and whisk this mixture until it becomes thick and coats the back of a spoon.
Once the lemon curd thickens (10-15 minutes), add the vanilla if using and remove from the heat to cool.
