Beat the butter and sugar with an electric whisk until pale and fluffy. Beat in the egg yolk, vanilla and a pinch of salt. Mix in the flour (you can use your hands or a wooden spoon to combine if the mixture stiffens up), then knead a few times in the bowl to bring together into a dough. Chill for 30 mins until firm.

Heat the oven to 180C/160C fan/gas 4. Roll a quarter of the dough out at a time between two sheets of baking parchment or on a floured surface to a 5mm thickness. Use a 4cm cutter to stamp out rounds (you should get 25-30 in total). Transfer to a baking sheet lined with baking parchment and bake for 10-12 mins until lightly golden and dry; they will cook more once the topping is on. You may need to bake them in batches.

Meanwhile, beat the egg whites in a clean bowl using an electric whisk until foamy and doubled in size. Or, do this in a stand mixer. Add the vinegar and, with the motor running, 1 tsp sugar at a time, mixing well between each addition until you have a stiff meringue. Beat in a few drops of green food colouring until it is evenly coloured. Spoon into a piping bag fitted with a wide star nozzle.

Reduce the oven to 120C/100C fan/gas 1/2. Arrange all the biscuits on a tray and pipe a tall meringue tree over each biscuit (hold the piping bag vertically over the biscuits to do this). Scatter with sprinkles and bake for 50 mins-1 hr. Turn off the oven, open the door and leave to cool for 1-2 hrs until the meringues have dried.