Comment by derpyzza

Meh, the rules for what an "X sandwich" refers to tends to differ by types of sandwiches. In terms of normal sandwiches, an "X sandwich" does indeed refer to X being in between two types of bread, like a chicken sandwich, or an ice cream sandwich, or a knuckle sandwich, etc. But on the other hand, sometimes an "X sandwich" refers to two things sandwiching some other thing, like a compliment sandwich — where you sandwich criticism between two bits of compliments, or a croissant sandwich — which refers to a sandwich that uses a sliced croissant as the bread. So calling it a "brain sandwich" works fine I think