Heat the oven to 120C/100C fan/gas 1/2. Line two baking sheets with baking parchment and fit a large piping bag with a star nozzle. Put the egg whites in a clean bowl with a pinch of salt and whisk to soft peaks using electric beaters, about a minute or two. Add the sugar while whisking, a spoonful at a time, whisking well between each addition, until all of the sugar has been added and you have a thick, glossy meringue. Whisk in the cornflour and vinegar, then enough of the green food colouring to create a light green colour.

Spoon the meringue into the piping bag. Start by piping the circular base, then spiral up so you have a Christmas tree shape. Put an edible star on top, if using, and sprinkle with hundreds and thousands. Repeat until you have 12 trees. Put the trees in the oven for 1 hr until crisp to the touch. Turn off the heat and leave in the oven for 1 hr to cool gradually.