  Spiral structures have been detected in evolved protostellar disks, driving
the disk accretion towards the central protostars to facilitate star formation.
However, it is still unclear if these structures can form earlier in young
protostellar disks. With the Atacama Large Millimeter/submillimeter Array
(ALMA), we have detected and spatially resolved a very young and nearly edge-on
dusty disk with a radius of only ~ 20 au in the HH 211 protostellar system at
submillimeter wavelength. It is geometrically thick, indicating that the
submillimeter light-emitting dust grains have yet to settle to the midplane for
planet formation. Intriguingly, it shows 3 bright linear structures parallel to
the equatorial plane, resembling a 3-layer pancake that has not been seen
before. The top and bottom ones arise from the warm disk surfaces, unveiling
the flared structure of the disk. More importantly, the middle one is in the
dense midplane of the disk and can be modeled as a trailing spiral arm excited
by disk gravity, as seen in evolved protostellar disks, supporting the presence
of spiral structures in the very early phase for disk accretion.
