In a saucepan, stir in the flour and the melted butter. Stir the 2 together into a paste over medium heat. Whisk in the milk to create the béchamel sauce. Drop the temperature a little and simmer the sauce for about 10 minutes so it thickens and the flour taste cooks out.
Put the gelatine leaves in two separate bowls of cold water and leave to soak for five minutes.
Put the yoghurt in a large jug, stirring to loosen it up and set aside.
Split the pod lengthwise into two halves and run the unsharpened side of your knife down the length of each of the pod halves, using firm pressure to take the seeds.