Heat the oven to 110C/fan 90C/gas ¼. Drain the chickpea liquid into a bowl (save the chickpeas to make hummus or use in a salad – see Here and now, page 14). Beat the brine on a high speed using the whisk attachment on a stand mixer, or using electric hand beaters. Beat for around 5 minutes, until very light and fluffy. It should have doubled in volume.
Add the cream of tartare, then add the icing sugar, 1 tbsp at a time, fully incorporating each tbsp before adding the next. When all the sugar has been added, mix in the vanilla and whisk until you get stiff glossy peaks. Transfer the mix to a piping bag with a plain or star nozzle.