Shake out elderflower umbels and wash briefly in standing water. Let the flowers drip off on kitchen paper. Remove flowers with a fork. Mix eggs, egg yolk, sugar, starch and elderflowers in a mixing bowl and stir over a warm water bath for about 8 minutes until thick and frothy.
Finely grate the rind, halve the lemon and squeeze the juice. Add lemon juice, zest and butter to the egg mixture and stir until a thick cream is formed. Pass the curd through a sieve, pour into a glass (à approx. 400 ml) and chill for approx. 3 hours. Keep refrigerated for about 2 weeks.