Whisk together the milk, maple syrup, cinnamon, egg and a pinch of salt in a large mug until combined.
Evenly spread the butter on one side of the bread. Cut the bread into bite-size pieces and add them to the mug, pressing down lightly so all the bread pieces are submerged. Sprinkle with the chocolate chips, nestling them between the bread pieces.
Microwave the bread mixture on high power for 2 minutes, pausing every 30 seconds. Let the French toast cool in the mug about 1 minute before serving.