Rub the butter into the flour. Mix in the icing sugar and vanilla sugar.
Knead the dough together.
Use one half of the dough as-is.
Mix the other half first with the cocoa powder.
Roll the doughs into lengths or roll them out into sheets. The two doughs can be combined in different patterns.
Leave the lengths to chill for about 1 hour.
Slice into rounds.
Baking time: about 7 minutes in a hot oven (200°C).
Notes
Instead of cocoa powder, the second half of the dough can be flavoured with pandan (using pandan paste or a few drops of pandan essence). This gives a green-and-cream biscuit with a mild, aromatic flavour.