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