Preheat the oven to 200 °C. Line a baking tray (30x40 cm) with parchment paper, pour the sugar onto it and place in the oven for about 5 minutes. Meanwhile whisk the egg whites into firm white peaks. Take the sugar out of the oven and lower the heat to 100 °C. Continue whisking the egg whites at high speed and add the hot sugar a little at a time. Add the vinegar. Now whisk at maximum speed for 5-10 minutes, or until the mixture is opaque and completely stiff. Click or pipe out small meringues. Bake in the oven for 3 hours, then switch off the oven and leave the meringues in the residual heat for a few hours, until they are completely dry.
Add flavourings as desired, e.g.:
2 tbsp lemon zest, yellow food colouring.
50 g desiccated coconut, half added to the meringue mixture and half scattered over the top.
3 tbsp freeze-dried raspberries, red/pink food colouring.
About 3 tbsp cocoa, 1 tbsp folded into the mixture and 2 tbsp scattered over the meringues.