  We study the torque arising between two corrugated metallic plates due to the
interaction with electromagnetic vacuum. This Casimir torque can be measured
with torsion pendulum techniques for separation distances as large as 1$\mu$m.
It allows one to probe the nontrivial geometry dependence of the Casimir energy
in a configuration which can be evaluated theoretically with accuracy. In the
optimal experimental configuration, the commonly used proximity force
approximation turns out to overestimate the torque by a factor 2 or larger.
