1. Place all the ingredients in a bowl and quickly knead into a ball of dough.

2. Chill the dough for 1/2 hour if necessary.

3. If all the ingredients, especially the butter, are rather cold, you can work the dough straight away.

4. Roll out the dough on a lightly floured work surface and cut out round biscuits.

5. Make sure that one half, cut out with a thimble, has a hole in the centre.

6. Bake at 200°C for about 15 minutes on the middle shelf until golden brown.

7. Leave to cool.

8. Spread jam on the bottom biscuit and place the top biscuit with the hole on top.

9. If desired, the vinegar biscuits can be dusted with icing sugar.

Tip: You can't taste the vinegar at all. It only acts as a leavening agent together with the egg yolk.