BackDoor 43, the smallest bar in the world

At number 43 Ripa di Porta Ticinese, you will find a doorway decorated with bottles
and shakers. Inside the doorway, there’s a sign on another door that
states “STOP! The smallest bar in the world”.
On your left is a small serving hatch in which appears a barman wearing a V For
Vendetta mask. He will either give you the key for the locked door (you need to
book ahead), or take your street drinks order. This tiny place of 4 m2 square
is one of the most incredible bar I have never seen.”