Combine flour, graham cracker crumbs, baking powder, oil, milk and sugar in a large microwave safe mug. Mix with a small whisk until no flour chunks remain. Make a hole in the middle of the batter and stick in the chocolate squares. Cook in microwave for about 1 minute. Top of cake should look done and surface of cake should be dry rather than gooey.
Either add mini marshmallows or one large marshmallow and place on top of cake and heat for about 20 seconds until marshmallows are puffy. Or you can also take large marshmallow and toast in toaster oven for a few seconds until surface is light brown. I think the best result would be to use mini marshmallows and melt on cake in microwave and then add a toasted large marshmallow for garnish.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.