Actually, it makes a butter sandwich. Since the butter replaces the peanut butter in between two pieces of toast, it's still technically a sandwich. If it were one piece of toast, then it would be a piece of buttered toast. :|
Anyway, it's still a MILD since you are trying to burn into your memory to do a RC. You don't have to use a mantra for it to be a MILD. A MILD could be as simple as trying to incubate a dream right before you're going to sleep, or as complex as always thinking your mantra and incubating at all times. MILD, put simply, doesn't require a mantra, just something burned into your memory that will cause you to become lucid within the dream.