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 peel, 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 and chill for about 3 hours.