In a small saucepan, warm the milk over a low heat for 2–3 minutes. Cut the vanilla pod in half and scrape the seeds into the warmed milk, add the pieces of pod and leave to infuse for 30 minutes. Discard the vanilla pods.
Put the flour, vanilla sugar, salt, eggs, orange juice and zest, and liqueur in a bowl. Add one-third of the milk mixture and the melted butter and whisk until smooth