In places that serve pancakes for breakfast in America, it is customary for three pancakes to be a serving. Those with smaller appetites, can order a "short stack" of 2 pancakes.
The joke here is that Leonard (?) is short, and that covering oneself with maple syrup as a sexual overture would be bizarre (and messy!)
There is also a built-in double entendre with "short" being a joke about a small penis.