In a food processor blitz the ingredients for the biscuits (if you don’t have a processor, crumble the cold butter into the flour, then incorporate the rest of the ingredients). Do not work the dough too much. When it is smooth, pop it in a plastic wrap and leave in the fridge for at least 30 minutes to set.
Meanwhile, make the meringue topping. Whisk the egg whites with a teeny pinch of salt. Slowly add the sugar and vanilla sugar, bit by bit, and whisk on high speed until you have a shiny, glossy, firm mixture that forms sharp peaks. This will take quite a while. At the end, add the vinegar and – if using – the chopped almonds.
Roll the dough out on a floured surface to the same height as a digestive biscuit. Use a round cookie cutter to cut out the biscuits and place on a baking tray. Repeat until all dough has been used.
Either spoon out or use a piping bag to get the meringue mixture on to each biscuit. I like quite a thick layer (so the meringue stays gooey in the middle), but if you prefer less, this also works.
Bake in the middle of the oven at 150 degrees for around 15 minutes until the base is cooked. I usually leave mine in the over with the door open for a further fifteen minutes,but this is optional.
Variations: add colouring, different flavours and experiment. This really is a lovely light treat and worth the effort.