One of the Pepperidge Farm Distinctive cookies.
This is another sandwich cookie, much like the Milano cookie. Unlike the Milano, the Brussels is a 3/8" thick, 2" diameter circular cookie. The chocolate center is much like the Milano's, although perhaps thinner. The cookie part itself is very thin and crisp. More similar to a potato chip than a cookie, although still sweet (and not salty).
The Brussel also comes in a mint variety, where the chocolate center has a white, minty additive. As this is identical to the mint Milano, it is likely the center of both cookies are identical.
Although it is eclipsed by the glory of the Milano, the Brussels is a fine cookie in its own right.