Mix the egg yolks with the mustard
powder, pepper, 1 tbsp cider vinegar and
a pinch of salt in a bowl. Mix the oils
together in a jug. Use an electric whisk
to whisk the yolk mixture, then very
slowly, drip-by-drip, add the oil. When
the mixture is pale and thicker, and no
oil is visible, swap from a drip to a slow,
steady stream – whisking all the while.
When all the oil is added and you have
a thick mayonnaise, taste and season
with more vinegar, salt and pepper. Stir
in the lemon zest and chives, then chill
until ready to serve.