Beat butter with powdered sugar into a smooth cream. Add milk, a pinch of baking soda or baking powder, scraped vanilla pod, or essence and flour. Knead a smooth dough, while adding a little more flour if needed.
Leave it in the fridge for 30 minutes. Tear small balls out of it, shape them slightly with oiled hands and arrange them in a tray with baking paper.
Put to bake in a moderate oven until they turn a pale golden color. Roll the baked biscuits in vanilla sugar (optional). Store in an airtight container.