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.

Store in the fridge for up to 10 days.