In a 1 cup or larger microwave proof mug, beat the cream cheese until light and fluffy with a small whisk. Mix in the sour cream, sugar, egg, lemon juice and cornstarch. Whisk about 2 minutes until light. Alternatively, the mixture may be mixed in a small bowl with a hand held electric mixer and then poured into the mug. Add any "mix-ins.
Microwave on medium for 2 1/2 minutes* (in a 650 watt microwave). Since microwave powers are different, you may need to experiment. You want the center to just start to bubble, and then stop cooking. If it is allowed to bubble and cook further, the mixture will "break" and you'll need to start over. Make a note of the successful cooking time for your microwave. Take out of the microwave and let cool to room temperature and then refrigerate until fully chilled, about 1 1/2-2 hours. If any liquid has formed, carefully pour out.
Sprinkle with mixed graham cracker mixture and top with any topping of your choice.