Place all blondie ingredients except mini marshmallows and chocolate chunks in a microwave safe mug. Stir well to thoroughly mix the ingredients together into a uniform mixture. Fold in a handful of mini marshmallows and chocolate chunks.
Microwave on high for 30 seconds. Remove from the microwave for a second and then microwave again for another 30 seconds. Repeat the process once more. Watch that the contents don’t rise too much and jump over the rim — if you see that happening, pause, remove from microwave for a few seconds and then place back for a shorter interval.
If adding additional topping marshmallows and chocolate chunks (highly recommended), then simply sprinkle those on top and microwave in a similar fashion in 15 second bursts (removing the mug out of the microwave in between each burst), until the chocolate chunks have melted a bit. Alternatively, you can torch the topping if you have access to a baking torch to get those marshmallows nice and toasted! Enjoy straight away (or reheat later).