I'd bet you would get better effects by buttering the bread on the inside, instead of the side thats getting the direct heat. If people are cooking edibles @ 325 instead of 350/375 to preserve the THC, i'd imagine the heat necessary to brown bread might have a little deterioration effect too.
Or even just make a regular grilled cheese with a little less butter, and then just spread some cannabutter on the completed but still hot sandwich. That might be the most effective way.
My favorite grilled cheese recipe:
The Inside Out Grilled Cheese Sandwich