1. Preheat oven to 170˚C and line a baking tray with baking paper. Also prepare a large pastry bag with a large circle tip (about 1 to 1.2cm) for easy filling later on.
2. In a medium bowl, whisk the egg whites until foamy, add half of the sugar (about 18g) gradually and continue to whisk until stiff peaks form, continue beating until they become glossy and smooth. Then add vinegar (or lemon juice) and mix with a egg whisk.
3. In another bowl, beat egg yolks and remaining sugar. Whip them on high until thick and the color is pale.
4. Pour in egg yolks to white egg mixture and lightly mix with with a spatula.
5. Gently add the sifted flour over the egg mixture and gently fold it in.
6. Fill the batter into the pastry bag and pipe about 8cm fingers, at least about 3cm apart, in rows onto the prepared baking sheet. Dust them with confectioner's sugar and let sit for a minute. Dust again for a second time.
7. Bake for about 15 mins or until lightly golden brown. Dust with confectioner's sugar before serving :) Store in a airtight container.