Immerse the Elderflower heads in water to remove any dirt or insects and dry in a salad spinner if you have one.
Place the sugar, lemon juice, zest and butter into a pan.
Snip the flowers off the Elderflower stalks into the pan.
Place pan over a very low heat and stir until all the sugar has dissolved (do not simmer).
Remove pan from the heat and allow the Elderflowers to steep for 5 minutes.
Sieve the contents of the pan to remove the Elderflowers giving them a squeeze with the back of a spoon.
Place the mixture back in the pan and place over medium heat then whisk in the beaten egg.
Once curd starts to boil cook for a further minute continuing to whisk (will thicken in this time).
Pour into hot sterilised jars. (Makes 2 x 250 ml jars).
Once cool, store in the fridge where it should keep for at least 3 weeks.