Place the graham cracker halves on a plate (or other microwavable surface).
Place chocolate bar (or chocolate pieces) on top of the graham crackers.
Microwave for 30 seconds. Check to see if it's the right melty-ness. It should be soft to the touch, but not liquid. Continue microwaving in 10 second increments until perfect.
Add marshmallow to 1/2 of the graham cracker/chocolate pieces. Microwave for 5-10 seconds until marshmallow(s) puff up.
Place the other 1/2 graham cracker on top. Smush. Enjoy!