Choose a microwavable mug. I like mugs that are a little bit shorter and wider rather than a tall skinny mug. Make sure it's big enough to hold at least 8 ounces.
Add flour, sugar, baking powder, cocoa and salt to mug. Stir.
Add milk, oill, and vanilla to dry ingredients in the mug. Stir.
Add 2 Tablespoons of the chocolate chips or chocolate candy bar and 2 Tablespoons of the marshmallows. Roughly crunsh half of the graham cracker and add it to the mug. Stir.
Microwave for 40 seconds. All microwaves cook a little differently so cooking times may vary. Since I prefer my cake a little on the underdone side instead of the overdone side I recommend starting with a shorter cooking time. I started with 40 seconds and it was just right. You can always cook the mug cake a little bit longer, I would cook in 10 second intervals until you get the desired doneness. Rember that in the next step you will be cooking the mug cake 10 more seconds. DON'T OVER COOK!
Sprinkle remaining graham cracker, marshmallows, and chocolate over mug cake. Microwave for an additional 10 seconds.
Remove from microwave. Let set for a minute. Drizzle with hot fudge sauce and top with whipped cream or vanilla ice cream if desired. Serve warm!