Biscuit Resonator
You don't eat it, you hear it
Biscuit
The classic blues guitar is the National resophonic instrument developed around 1930. These instruments are powered by an aluminum cone surmounted by a wooden bridge over which the strings pass to deliver their vibration to the cone.
Here's the cone:
That black wooden disc on top is called the "biscuit," and the assembly is often called "biscuit cone." The bridge is the vertical piece in the middle of the biscuit.
A few people (probably Oreo eaters) call it a "cookie."
From the outside, the biscuit can be seen by looking under the palm rest in the middle of the cover plate:
It's a little dark under there, but you get the idea.